Polyurethane foam does oxidise - just leave a piece of the yellow foam from Clark Rubber in the sun for few weeks and it turns brown. I assume the black camera foam would do this as well but it's a bit difficult to see.
I have a stack of this black foam and some unused boxes that I was intending to use for eyepieces etc. I thought about using the closed cell polyethylene foam but had trouble tracking down a retail source.
I bit of browsing turned up
http://adfoam.com.au Besides signs, they also make foam packaging and inserts. They also sell Pelican and Storm cases. [Doh! just noticed this in a previous posting in this thread]