I would consider using a tooling board instead. this is very easy to machine on a router (easier than XPS i would think even) and you don't need to deal with primers and fillers and all, so all in all its not much more expensive i would have said. also going to end up more precise.

If any of the two i would clearly choose XPS, but if it needs to be cheap i would make it from MDF, mill it like a half a mm oversized, soak it with two coats of epoxy resin, machine to final size, coat again with resin, sand and "polish". Make sure to coat all sides of the MDF in this case, so it absorbs humidity uniformely and doesn't go all banana on you.

hope this helps.