The model railroad guys have been doing this for about 20 years now. You cannot use straight urethane for casting as it flakes and shrinks after about two years. I have seen the older urethane kits and for most purposes they are junk now.

They also vacuum down the blue silicone to remove any possible trapping of air bubbles which then show up in the surface of your finished product.

There is some stuff with fine metal inside which is a bit more expensive, but takes the heat and stays pretty rigid. It will also cast to very thin sections and not break upon knocking around.

I would suggest that you investigate with those of model railroad interests.

Wm.