https://www.ecfibreglasssupplies.co....3-mm-sheet-wax
Wax sheet is a bit pricy but the best way, you can form curves with ease. It is lightly self-adhesive and will form easily to curves, it is strong enough to hold to keep its shape on flat panels.
Backfill with some gel coat and then use chopped strand mat and overlap. This way you won't suffer shrinkage in the future,
After removing the wax you can fill any imperfections with gel coat.