Wood will work but you indeed have the problem with the tree sap. That can
be cured by putting the mold in an oven and cooking it for a while to get
the moisture out of the wood.
Another is to use plaster or pour stone. This also needs to have the
moisture cooked out of it before usage or you will have shattered molds.
Note that plaster will degrade with high temps as the water is driven out of
the chemical bond with the plaster itself.

--
Why isn't there an Ozone Hole at the NORTH Pole?