I've been playing around with new buttons but from a different angle.

I'm using some stuff called Sculpey to press into the molds. It's not transparent but it does raise some interesting possibilies since it comes in different colors (including 4 glow in the dark colors), is cheap and doesn't have a problem with bubbles.

You can buy it at any craft store (Fimo works too). It doesn't provide the true back lit buttons but it's another option. Just make sure you make your mold out of a silicone rubber that can take the heat (265 F).

I used Smooth-on's Evergreen 40 rubber. It's pours bubble free but has a low melt point (170F which I didn't find out until after I made the mold). If you want to do resin casting, it seems to work well and is a 1 to 1 by volume mix ratio.