Easy. Make a 3D model of it yourself.
In all seriousness, though, as long as you can get a good look at it with accurate measurements, it shouldn't be too hard to make your own model in Blender or Maya.

 

I did a little looking and found nothing.

looking at them it looks do-able. I would try modeling one myself but without the n3ds or a faceplate it would be hard and I wouldn't know if it worked.

if you could get me some dimensions I might try but I have no way to test them... and I am not very good at modelling but I do have a printer so I could at least get a proof of concept.

out of curiosity do you know what printer your friend has?

 

General dimensions aren't an issue, it's the curve on the inside. I haven't got anything on hand to measure it with. I'll see if I can find a protractor tomorrow (it's 4AM here) and I'll let you know what I come up with.

 

I would KILL for a custom Waluigi set...

 

I know this topic is a little old, but we 3D print blank plates for the new 3DS - We've also made a few simple custom plates too http://www.etsy.com/shop/chompworks

Only the top cover is in the store currently, we've also modelled the base but ccurrently trying to source the little screws.

Here's a sonic plate we made for someone this week -