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 https://youtu.be/DrqdHVeBkp4
Google’s ongoing Android lockdown feels like the end of an era, with the understanding that eras don’t end overnight. They fade away slowly.