i would advise coating the foam with something first, resin will eat most easily available expanding foams, you need a special kind for it to hold up.
i would say try to make it as perfect as you can. you can fill in holes etc, but if you spend the extra hour making the shape nearly exactly how you want it, you'll save 10hrs of applying kitty hair and sanding.
you could try the expanding foam, then apply some kitty hair and sand, make sure the kitty hair covers everything. and then apply some release agent and glass over top. the best way would be to use that as a negative mould, but you could also use it as a one off final project. if you want to go that route i'd suggest sticking with cloth all the way as it will require less filling afterward, and therefore the shape won't be distorted as much.
kinda off topic, but i've found using mat is way easier for corners etc. rip it, don't cut it, and dap, don't stroke. the paint roller works wonders as well, but can get pretty messing if it starts to stick.
