I noticed at my local Staples store they were carrying PALM/Visor clear screen protectors. I don't remember what the price was, but they were designed to self stick to the screen (without adhesive, probably static) and would be perfectly sized, eliminating the need to make your own covers from overhead plastic.

Those would be a good base for stick-on tactile buttons. You get several in a pack, and each could have a different button layout, as previously suggested.

Dave