I know what I know pretty well, but I also admit I definitely DON'T know what I don't know. I also have nothing to prove. That said:

I haven't attempted any class A surfaces, only structural repairs + "filled in" the underside of my 6 pack scoop to allow me to make an air pan. So this might not apply to class A but maybe it will be useful.

I made my mold out of cardboard covered with regular, clear box sealing/shipping tape. Some digging online revealed some statements that the resin would not stick to the shipping tape, which turned out to be correct.

No way would I suggest you could just cover the bumper with tape and go to town. But, maybe that idea might be enough to get started, make a huge mess, breathe some awful fumes, and could maybe be refined after that.