Yes, I'd recommend Tap Plastics
urethane resin. It's 2 part by volume, the recipe is pretty forgiving (no weighing, eyeball measuring graduated medicine cups works fine) and the finished casts are, presuming you mixed correctly, rock-hard.
I've used Smooth-On Silicone, but never their resin, so I can't speak to it specifically, but are you sure you blended it correctly according to the instructions? Some of them, such as Alumilite, will "work" at room temperature, but are a little soft-ish afterware. Not a game-breaker, but not totally rigid either. And, of course, that's what you'd expect, since that's not following the instructions - Alumilite is supposed to be added to a pre-heated mold. I prefer to mix it and then place it right under the heating vent for about 20 minutes, casts are like stone. So, you sure you're not missing some quirk like that?