There are a number of materials available to make tooling that is soluble in water or other media.

google "soluble tooling" and there are 42000 hits.

good luck

Composites and Airplanes - what was I thinking?

There are gremlins in the autoclave!

ive used axpandable silicone bladders over a low melt alloy core it works really well

I have use extruded styrofoam, and then melt it with acetone or gasoline, but a good result depends on what size a piece you are making.

another way I do it, first a waxed balsa plug, then a thin layer of glass and epoxy, when split open this shell, take the wood out, glue it back together, and laminate over this shell.

contact me if you want to talk more about this, I don´t check eng tips too often.

Try Aquacore at

http://www.acrtucson.com/products/Aquacore/index.htm

make sure that the temperture you need to melt is above your cure temp and below the working limit of the material.