You really shouldn't be casting anything with Plasti-Dip, IMO. This stuff is made to dip tools into, so you can coat the handle with a better gripping plastic layer. I once tried to dip a custom-made lightsaber (full size, not a 3-3/4" version) into this crap, instead of trying to work out a better solution for a hand grip. It was ****. I ended up peeling it off a day later and throwing the can in the garbage.
Good luck if you go that route.