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Mate, I know you like to take the path of most resistance, but you could be having an easier time with your mould tool making.
Rigid foams are brilliant for the job and the blue Styrofoam that you have got is one of the best.
Filler is awful as a surface substrate.
Skin your foam over with a layer of fine woven glass cloth in a medium of either, Water Based Polyurethane Varnish or, if it doesn't dissolve the foam, Epoxy.
Polyester resin WILL dissolve the foam.
Skinning with glass cloth will make a unified surface over all the materials in the mould tool.. Foam, ply or whatever. It will add less than a mm to the overall size and retain all the detail.
It also isolates the foam from any solvents that you might apply later, that might be contained in any filler primer or other coatings. You could use epoxy on top of the skin to build a finish, but I think it's rather hard to rub down? I use water based poly varnish with lightweight filler mixed in, to make my own filler primer and it's very easy to rub down indeed.
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