Hi Jerry

Any of the strofoams work-either beaded or solid-and you can get it at a
hardware store as insulation.  The blue foam shapes easily and is cheap.  
However my preference for plug making is wood.  The problem with the "lost
wax" method of laminating is that you end up with a God awful bunch of toxic
sludge and the back of the part that was against the part is left very rough
and requires a lot of cleaning to get rid of the residue.  A slight variation
I did use once was to tape over the plug and once, I also stretched some
Saran wrap over it.  That also allowed me to use polyester resin for the
part.  

If you want to go fancy, build in wood and finish the surface with Ram primer
or Featherfill, both are polyester based spray or brush on sanding primers
that can be sanded and buffed if you want a really nice part or just lightly
sanded and waxed.  The parts come out great!  And you can see how deeply the
wax "soaks" up into the paint leaving it darker.  However, just plain wood
releases beautifully when waxed and can leave a decent surface although some
grain will come through.  One trick to eliminate that is to brush on a
sanding "gelcoat" of epoxy.  All you have to do is mix a little micro
balloons and fumed silica (Cabosil) together in about 25/75% volumetric
ratios with the epoxy until it is just shy of being thixotropic, (leaving it
just a tad runny will allow it to level itself a bit on the surface), then
brush on to the surface.  Once stiffened up you can continue the lamination
then when popped off of the plug, you can do some sanding without getting
into the structure.  And if you don't go into the fibers and want to paint
it, you can do so without the prime/sand step.

I always keep a gallon can of Featherfill in  my shop since I build a lot of
molds and plugs.  I also keep around a few sheets of melamine finished
particle board for plugs or molds since it is cheap and easy to work and
requires no finishing in order to be ready to wax and laminate.  In fact just
this last week I used both to build a keel trunk for my boat and will be
using more for the new engine box.  Since these were all basically flat
sided, I only used the Featherfill to finish the raw edges of the
particleboard.  The parts came out perfect as if they were from a highly
finished mold but in fact it was a one use, then toss away plug.  I've also
used the melamine board as a mold for making flat cored panels.

Dan Newland