Oh, forgot to ask, what's the best way to block off the opening for the wheel so that I can begin making the mould?
I would not bother to seal the hole. I would just stick some masking tape inside trhe pant around the edge. When applying the gelcoat go up to the tape. The hole will be cut out anyway.
Do not take chances with mold release wax. I use only wax that is specifically meant for mold releasing. I will not take a chance with any other product.
Ed S