There are any number of 3d printer services that you send a file to and they send something back, or hacker spaces/maker spaces with that as a service, or people that rent out their own 3d printers, CNC machines and vacuum formers.
https://www.retroasia.com/products/panasonic-q-system-jp-us-modded-with-component-cable
https://www.ign.com/articles/2000/12/22/gamecube-from-every-angle
https://www.nintendoworldreport.com/feature/27664/nintendos-expansion-ports
and some ebay listings for replacements for the gamecube sourced me enough images that I reckon I could make a stab.
Thumbnails, click for larger.
Can't find a shot of the Q serial 1 cover from other angles in a casual search sadly but with the pictures there I don't anticipate any real problems as long as you consider the internal ribs inside the slot (just left of the finger holes for the spring clamps in the pictures) when it comes time to the box section if replicating the gamecube approach. The box and ribs are mostly there for stiffening and alignment anyway (a sensibly thick* bit of plastic would probably still flex without any ribs or might be able to be twisted out of the slot under the little kid treatment) so you might be able to get away with rather less than that
*you have a limit here as that shelf between the true outside which forms the resting point of the cover is rather thin. Make the base sheet too thick and it will stick out.
I would also note Nintendo's CAD files did leak as part of the gigaleaks. Don't know if this would have included the Q (which might be Panasonic's own setup, and if it was different because it was Panasonic responsible then who knows). Even if it does it might be more hassle than it is worth, but I don't know on that one.
I also note that
https://cults3d.com/en/3d-model/game/gamecube-serial-port-2-2pieces opts for separate spring clamps and the way they attach makes me think they are a weak point for them to have that much reinforcement/mitigation for internal forces.
If I were doing it and using a thermoplastic (rather than thermoset) I might print or form the side bit flat, and then heat it up after placing it on a suitably curved bit of wood or something so it takes the curved form rather than hoping my 3d printer can handle it. Pro tip in that case is make it longer than you need so you can file off a bit to have it fit nicely. If I am doing vacuum forming (can glue the internal box section and reinforcing ribs on later) then probably go for the curve ahead of time, though whether you will readily find a vacuum form setup capable of doing something as thick as you probably want for this I don't know.
As far as designing this in cad. The curved section might be tricky for some approaches but this would be a wonderful beginner project -- something you want to happen but nothing too terribly tricky (design the base bit of plastic and then in the CAD then on the relevant surface extrude any box section and reinforcing ribs at suitable locations for a suitable distance, assuming you don't make them separately and glue the box section and ribs into the simple sheet you cut and heat formed).
