I'm sure someone else here has more experience with this, but:

I've done this by covering the foam with body filler (Bondo in the US) and sanding it smooth. The big problem with foam is that it's porous, so you need a way to seal it so that your gelcoat doesn't penetrate and mechanically lock to the plug. Technically, you could just cover it in plastic packing tape, but that's going to leave lines in the mold that would have to be addressed. Do you need to keep the existing part or is it possible to sacrifice it?
A picture would help.

Has anyone taken a soft foam and somehow turned it into a mould?  Basically I want to somehow take an existing foam structure and make a female mould out of it.  I can post a picture of what I'm talking about if it helps, but it's a the box my headphones came in.