I've fixed a couple of keyboards using hobby shop track bed foam, but have not been very happy with the "feel". That foam is considerably less resilient than the material Keytronics used. After trying a number of alternate materials, I have found something that is almost identical to the original. Take a look at eBay #171549720453 for an example. It's 5mm thick with an adhesive backing, sold for shoe inserts. I stuck 0.075" plastic sheet to the backing, then turned it over and used spray adhesive for the aluminized mylar. Worked like a charm. Just finished resurrecting a N* Advantage and the typing feel is (in my opinion) far better than the last unit I did with track-bed foam (Corvus Concept). Steve --