I CNC machine little metal plugs that fit into recesses in bodywork on boats.

Well, I have a customer who asked me to do a custom size - but it's not feasible for him to send the boat to me, obviously, so instead he took bondo and made an impression of the recess. So I have in my hand a piece of hard bondo that looks like a mushroom - the underside of the mushroom top would be the bodywork contour and the stem of the mushroom is the plug I need to make for the bodywork.

How would I "re create" the bodywork from this piece? I want to put some sort of mold release on it and use some material to make a negative of the part (which is itself a negative of the bodywork) - so I'll be left with an approximation of the original.

Whats the best material to use and how do I do it? I was thinking fiberglass but the only issue is it may not come out smooth when I am done. The Bondo is glass-smooth now and I want me negative-image of it to be smooth too. Also, what do I use to ensure it doesn't stick and I turn it into a solid mess

When I get the negative of this part, I will mount it in the CNC machine with a probe head to measure it, then I'll machine the plug and use the part I cast from his plug to ensure a correct fit.

Tolerances are huge because this fits into molded plastic pieces, which have sloppy tolerances, so I don't need to be within thousandths or anything.

Thanks for any tips!