Scratchbuilding Vehicles

OK here goes.

I want to scratch build some vehicles for SG but I want to be able to
make
molds from them when their done so I need to know what is the best stuff
to
use to make the masters from, I've tried the DAS white stuff and am not
too
impressed with the result so far, or maybe i'm doing it wrong, certain
parts
seemed to shrivel up as it dried.  I have several designs on paper and
in my
head but need to know what is best to use for the making.  Any help much
appreciated.

Paul Owen