Just note that if you 3D print something, if you use the wrong material, there’s a chance it may melt.

I would use some kind of CAD software to design a backplate, making sure to add things like physical supports with screwholes that match your motherboard's mount points and vents at the right spots. You can get things like that 3-D printed, would be an inexpensive way to prototype it :)

The absolute easiest way would be to 3D print the whole thing, leaving slots for steel rods to reinforce the frame.

The cooler way would be to just 3D print the rim and make the back and front plates out of laser-cut aluminum. There are laser-cutting services if you don't have one of your own.

If you don't have access to a 3D printer, you might consider brass. Unlike aluminum, brass can be folded or hammered into shape, so the front and sides of the tablet could be made from one folded and soldered/brazed sheet, with ports and vents cut with hand tools. I wouldn't call this easier though -- you'll need some practice to keep it from looking sloppy.

I'd recommend taking some inspiration from cyberdeck builds and other custom electronics enclosures. https://youtu.be/qzEd50uzdF0?si=6Bk1-QPlVcoNRcVO https://youtu.be/DrqdHVeBkp4?si=1sqfqUsp66He2bS5

Any plastic sheeting that's easy to bend with or without heat could be a decent option that doesn't have high tool costs/skill requirements.