> I'm fairly decent with clay, always wondered if I could make something like a raspberry pi case with sculpey.The short answer is yes you can, but it's a PITA. Among other problems you have to deal with shrinkage and drooping. I'd only use that approach for a quick and dirty prototype that just needed to hold parts together and survive rough handling.
A better option is to build a case from parts you cut out of a sheet of plastic (anywhere from 1mm on up) that you then glue together. Once built, rounding the edges will make it a lot nicer to hold, but it will still basically be a box, and the glues you use are generally pretty toxic so don't breath in the fumes OR get it on your hands.
More involved would be making the case by vacuum forming a heated sheet. The forms (one each for the top and bottom instead of the two you need for injection molding) are typically hand sculpted or machined from wood, filler and primer rather than metal, and vacuum forming is something a hobbyist can accomplish at home with a DIY setup. You can get a case nearly indistinguishable from an injection molded part, at least from the outside. The more complex the shape the more labor intensive the fit and finish will be (for example, getting the edges of the case halves to match up).
However you make the case, there would be a lot of keyboard holes you'll need to carve out with something like a dremel rotary tool. It might be easier to make the case with a single large hole that you glue a thinner insert into that will be easier to cut holes in with a hobby or utility knife, but I've sliced my hands up a lot when doing this sort of work. A rotary tool seems scarier if you're not already familiar, but with as little as 30m of practice you'll probably have much better control than you would over a knife and lower risk of injury (as long as you use eye protection, etc. You really don't want the plastic shavings flying around getting into your eyes).