Looks like a cool airplane!
Some advice from somebody who has done a couple of plugs like the one for your cowling: Don't scrimp on the fiberglass skin for the plug. Go with six or eight layers of 9oz cloth or so. If necessary, sand the foam down a bit further to allow for the extra thickness of glass (about .011" per ply). Also, use an even mix of 0-90 and 45-45 fibers. Once you have a nice thick-skinned plug, then worry about fine contours and surface finish.
The reason to use such a thick skin is to keep the exterior surface smooth while the stuff inside it (the styrofoam and spray foam and wood) tries to change shape and shrink and grow. It doesn't change shape much, but when it does the different materials will all do it at different rates, giving what we came to know as the "Michelin Man effect."
If you make a nice smooth, well-fitting plug, you'll get a nice smooth mold, and that mold will yield parts you'll be proud of. Email me offline if you need any help. Experience is what you get when you don't get what you wanted, and I have lots of composites experience.
Thanks, Bob K.