I just posted about looking for a GBA size cart, too, but this is intriguing. Has anyone does this successfully? It sounds relatively simple, but I don't have a good feel for exactly what's involved. Is it roughly just unscrewing the case it's in (the lite case) and re-using an old GBA case? Anything else needed? I'd love to hear from someone who's done this successfully! Thanks.
I have. I had to order an NDSL EZ from DX because they're cheapest and they don't sell GBA sized ones. I planned my modding long before the cart arrived but I found it impossible to use my plan once I got it and saw the real deal. I was planning to simply slide the circuit board into a GBA cart's case, but it was not possible because of one important issue.
The EZflash's case back is deeper than a GBA cart's. This is because the circuit board has a microchip on its back which needs extra room, else the board sticks up too much to get into the GBA connectors. This is where the trouble begins and you start looking for a way to
use the NDSL cart back with a GBA front.
And this is the result, blurry and dark as it may be. A cartridge consisting of the front of a GBA cart, the top side of the back of a GBA cart, and the back of the NDSL cart. The top of the GBA back needs to be joined with the NDSL EZflash back so that their size adds up to that of a normal GBA cart. this means
cutting the GBA cart back in two, preferrably with a hot knife. Be weary that the plastic may mlt away far deeper than lanned and you may end up with too short a GBA top like I did. I had to fill in the gap with paper and then use powerful glue to bridge the two. The glue impregnated the paper perfectly, and turned it solid like wood. The top GBA and NDSL back are now joined as one piece, each with their own screw.

I did some nasty crap which I should not have done to the front. I had issues making it fit in, doesn't exactly slide, so I tried to melt away the sides' thickness. Now it looks like it was bitten, and didn't fix anything, the problem I found to be with my homemade label, which was cardboard cut off the box itself and was too thick.
The second major issue I had was once the cart was done. The top and back don't exactly fit flush into one another, they're different size by a milimeter, so the cart "caves in" a little if forced. It's not unstable, and the two screws help with that. It stays in one piece but "bends" a little if inserted fully into the slot. During play I found the game to suddenly freeze, along with a repetitive garbage noise, just like when you take the cart out during play, clear sign that connection was being lost. This was because the bend was so sensitive that even pushing down on the NDSL case with your fingers when you press buttons could cause it to disconnect. The solution was adding 3 "teeth" so the cart doesn't go completly in, and the freezes went away. This problem only occurred in the NDSL, not the GBA, because the GBA is able to fully fit the cart in, not allowing it to move or bend in any way.
You can clearly see the solidified paper just on top of the battery. Sorry for the blur I only have a mobile camera. You'll notice a white patch on the right, where the rumble motor used to be. I physically removed it off the EZflash as soon as I discovered what a horrible experience rumble is. No it's not the EZflash's fault, their rumble is in line with the official one. But it's noisy, the vibrations, far from immersive, are disruptive, eats battery power like a monster, and it makes your ears bleed and remind you of the painful experiences you may have had at the dentist, I swear this thing sounds just like that darned drill. Back to the topic,
the rumble engine is not necessary to remove, I had because I originally tried to put the EZflash into a GBA cart. However if you want it removed it is easy, just slide a thin screwdriver under it and dig away the black foam.
Oh god I'm only now seeing how blurry these pics are........ aaaaanyway this is the inside of the front GBA case. It joins the "frankenstined" back of the EZflash and fits perfectly once screwed into place tightly. You may see a gash on the right with a bit of eye straining. I tried to make extra room for the battery when I didn't know why applying pressure to the cart made it freeze(mentioned earlier). One of my mistakes, just like digging into the sides. But on the left, if noticable at all, is the socket into which the NDS screw goes in. I
ripped that socket of the EZflash's original NDSL front and glued it onto the GBA front. You can mark the spot by assembling the case and inserting a screwdriver through the hole but the best way is to screw the socket in, apply glue to its butt, and assemble the case, then you'll know for sure it gets glued in the right spot. Do not screw it completly in because when unscrewing the force might make the socket glue come off, the glue is only temporary for you to get it to stay in the right place, then you disassemble the case again and glue it where it's stuck.
Glueing a socket isn't the only thing you have to do to the front part of the GBA case. You will also have to cut away a piece of plastic in the plastic "wall" which stands where the connectors begin to make room for the SRAM battery...
And this is the final result -- a GBA fat cart that sticks out of your NDSL. Yaay. Looks ugly but at least it'll work in your GBA. I covered my NDS' serial number from view god knows why... I've seen everyone do it but I honestly have no idea why the heck. I mean are people supposed to somehow find you and come up to your door and rape you, or perhaps Nintendo throwing the book at ya finding out who you are or banning your console off wi-fi? I don't understand...
Also it works with the GBA, although it sticks out half a centimeter. It's because of the "teeth". Not really that annoying. Again serial number concealed for great justice... yaaay.
Aaaand as you can see my monstruosity actually works. Glory glory glory.
Bottom line: getting an NDSL to fit into a GBA case can be done. Even though they're different builds altogether the circuitry works fine in a GBA. But:
-you will most likely have to sacrifice a GBA cart case
-rumble motor glued onto the case though easily removable; if you don't wanna remove it you'll have to keep the whole board ONTO the back case so "play nice" while you're doing all the glueing and cutting around there
-there are minor size differences that may make the case unstable unless it uses two screws(one in the GBA case and one in the NDSL), so be ready to rip the screw socket off the original NDLS case and glue it onto the GBA case
-don't try a full GBA case - the back lacks the necessary depth because the circuit board has a fat chip on its back
-same stability issues may need to be addressed by preventing the cart from going inside the last half a centimeter
-you will have to cut through a GBA case. don't go all muscle man, because you WILL need to use a hot knife to cut thrrough all that hard plastic
-careful when melting plastic, gashes are deeper than they may be planned for, especially once you get half through
-cart may need heavy polishing with a file on its width else it will fit in VERY hard and even harder to take out. DO NOT HAVE THE CIRCUIT BOARD IN AT THAT TIME, the plastic powder can get in and do some pretty nasty stuff
-powerful glue strongly recommended, preferrably the type whose vapors hurt your eyes... guess that also adds masochism to the requirements
-make a hole in the plastic wall of the front wall of the top case, just like the EZflash's own, so it'll have room for battery
-around 10 hours of work required, complete with testing and retesting
-it will look like crap
The only pro I think it has over a genuine GBA sized EZflash is that it's lighter by a gram or two because the circuit board's smaller.
EDIT: Did another thing to it today. Bought a fat CR1620 battery to replace its internal CR1220 SRAM battery. CR1220s have 35mAh capacity, a CR1620 has 70 so that's double the save lifespan, should last me 6 to 8 years instead of 3 to 4. I feel sorry fot the left side of the cart right now. Underneath it's got a fat chip pushing it up and now it's also got an even fatter battery on top pushing it down. The whole side has "inflated" though amazingly it doesn't make it any harder to insert or pull out than it already is. However, now with double the SRAM battery life I can finally brag that my freaky-looking cart is in one aspect superior to your average EZflash. Nyahaha! XD