Depending on what's available to you locally, I suggest using particle board with a melamine laminated surface. Think kitchen countertop material.
The laminate is non-porous and is less likely than wood fibers to become scratched or dented and thus disrupt the mold release layer.
If the partcle board laminate isn't thick enough to withstand the casting pressure, back it up with plywood.
Or just buy the laminate material and laminate it to plywood or particle board at the thickness/strength you require.