What if you use some sort of "impact proof" resin? I only know a dealer here in germany who sell that stuff.
It's called "Task" series.
Links: (Sorry only in german available ;()
http://www.kaupo.de/Produktkatalog/Polyure...TASK-Serie.html
"Hauptanwendungen sind Prototyp- und Architekturmodelle, Spielzeugmodelle, ultradünne mechanische Teile und Spezialeffekte."
"main application is for prototyping or architectural models, toys, ultra thin mechanical parts and special fx."
I used that stuff for grip shells on my first E11 Blaster series. That stuff is awesome, you can't break that stuff and it's lightweight. A little bit pricy compared to nomal resin, tho. But it's worth the price and you can use less of that stuff to get a durable cast.
Cheers,
Joerg / TK-607