I have done a few cowls, the last one was for a .60 pattern. The usual method, Stick a foam cube, mark the exact thrust lines, stick a 3/32" lite ply mouth ring matching the spinner back plate taper lines. That and a few card templates made from the plans give me a great guide for carving the foam. Coat the plug with thinned white glue (sand finely between intermitent coats). This also gets the foam surface hard. Once sanded smooth, I overlap the fuse and cowl an inch either way with a clear cello-tape (which takes great care for the release from the fuse). Now I haven't done much experimenting, but so far a medium wt. Great PLanes glass cloth (single lay-up)is all that I have used. I run a thin coat of epoxy to soak the weave and after that dries, a thicker coat of epoxy. I heat the epoxy to be able to brush it on nice. After curing, I use balsa filler mixed with dope to keep it light and then sand it down.
Now, its a good time to mark the cowl openings. Usually I stick a syringe with thinner to burn the core near the fierwall till I am able to release the cowl off the fuse. Dig the foam from the inside. This is the point where I decide how much additional stiffening is required, depending on the flex and the openings. So patches of GF are applied. Next is to cut the openings.
To get a better surface finish , I cover the cowl with Monokote, works great with good heat and keeps the cowl light as well. It surprises me how well it goes over curves and minor dings don't show up at all. Its easier to do this with the cowl back on the fuse.
Thats my pennies worth, hopefully.
Good luck.