Actually, it's pretty easy to put an A500 m/b in an old AT server or similar large tower case. You need either flying leads for the ports out to the back of the case or use a nibbling tool to make a cut-out large enough in the back of the case for the ports to stick out. The keyboard is really not a problem, just make yourself a keyboard cable out of some old monitor cable and attach it to the DIN on the m/b. You then can mount the original keyboard in a homemade case and connect the cable to the original flying lead. You can power the m/b either from the original p/s or cut the square power lead and wire it from the AT p/s in the tower. It's been done.
You're probably going to want to add a faster cpu, hd and ram though, so look for a gvp or other board and leave enough room in the bottom of the case to install it.
