If you want to try RenShape, let me know. I have some scraps. I could send you a little...but just a little. I'm stingy. :-)
Also- a way to avoid having to smooth out a somewhat porous material is to make your buck just a little bit more undersize, then vacuum some .010" or .015" over it, then polish up the plastic copy. A lot easier than getting a glass-smooth finish on putty or RenShape.