Just casually, my observation so far is that the 7200+, 8600/9600, and Beige G3 (both kinds) all use the same feet. They're basically just sizeable chunks of rubber. It's probably not really worth making it as a specific product, but a wiki idea would definitely be highlighting some suitable replacements, and perhaps measuring the depth of anything protruding from the bottom to see how big you really need the feet to be.
You could, for example, fill each of those spots with two smaller "button" type of feet, instead of bothering to cut a big rectangle, or just slam larger circular feet on there.
I still have the feet I linked up-thread several years ago but haven't put them on anything and, I have, in fact, forgotten why I originally bought them.
The feet for things like early-mid '90s Apple monitors and stuff like the Mac II family are probably in worse shape (other than just falling off) and need more specific cutting, but I don't know off hand if Apple predominantly used a single size or if there's anything like feet sticking up into chassis as attachment mechanisms going on there.