The feet for the keyboards are silicone. They aren't particularly special, just custom. Here is a picture of the mold for the silicone feet. It's a good quality mold, but it's just made for small scale production. I needed under 100 pieces so there are only 4 cavities in the mold. A production mold might have 100 cavities. 

Silicone can be difficult to glue or tape in place. Make sure you get good tape. I'm using 3M 9731 double sided tape. It's one of the best for sticking silicone to things.

Here are the feet installed on the keyboard. This isn't white silicone, but silicone with no color added. It would have wasted a substantial amount of material to make both white and black feet so I chose the neutral color. Since it's on the bottom of the keyboard, I don't see it as a big deal. If I had made more keyboards, I would have gotten both black & white parts made.