@Samuel - after re-reading your question/post, I wanted to add some clarity.
The rubber I am currently using is the 6mm gumlite material from Vibram. This is what they call an expanded rubber but is essentially a mix of rubber & neoprene (hence the reason you get a slight impression under your foot after wearing them a while, it's like you get a pair of custom custom sandals, lol).
However, I have been in contact with Vibram direct trying to figure out some other options for R&D purposes as well, just trying to continually improve the original. I have another material on order that should be in next week, enough for a pair or two, but am also working with Vibram to see what they recommend. The important thing for me and the sandals is that I need a compound that I can heat and mold. I prefer the sandals to be hand-molded around the edges and under the arch to keep the sandal right under your foot and also help prevent the walking-in-tall-grass-front-of-the-sandle-rolls-under-foot syndrome others have complained about. So far, with the molded footbeds it has worked out really well for me.
Hopefully that helps.