I have never used Devcon rubber substitutes, but have had great success with casting urethane elastomor. They are available as 2 pack products in multiple duromiters and colours from plastic suppliers down under and I guess world wide. Moulds can be wood steel silicon or anything that is release treated. Heres at least a starting link that may help.
Graeme
http://www.casspolymers.com/adtech/a...c_urethane.htm