The rubber i used from RS is Devcon Flexane 60. And the primer is Devcon FL-10.
http://uk.rs-online.com/web/search/sear ... &R=0692845
http://uk.rs-online.com/web/search/sear ... &R=0692895

In the link below I have some pictures of the wheels I made. I used gypsum but it was tough tp remove it from the rubber. Maybe spraying some sillicon rubber on the gypsum would have helped. The blue mold is sillicon rubber but it cured too fast to make 5 negative moulds at the same time. That was a waste of money and the gypsum was a cheap alternative afterwards After removing it from the gypsum i grinded them on a lathe to make the wheels smooth, round and concentric with the bearings.
http://picasaweb.google.com/Jeroenvdlo/Firestarter1#

I hope I will have permission from John Reid to post 2 of his pictures in the link below.
http://picasaweb.google.com/Jeroenvdlo/JohnSWheels#
He used wood and sillicon spray and it seemed to work well. I am still curious how long the wheels lasted in combat.