Similar to this -> http://makezine.com/2012/04/12/the-most-beautiful-homemade-d...

I think its a combo of two compounds that harden when mixed.

http://uk.rs-online.com/web/p/potting-compounds/1991402/

It's just a base + a hardener, mix and cast. Make sure you protect switches and connectors before casting. The reaction is exothermal, if you cast larger volumes you may need to cool the whole thing to avoid trouble (such as fire).