[EDIT ADDED -- seems like plastic worker gave a much better description than I did -- for some reason I didn't have but 2 posts showing when I wrote the following -- go figure] And yes use the vacuum technique!
It sounds like you want to make a female mould. Then do your product layup inside?
Have you considered using the cnc to route out 2 FEMALE halves of the fuse and layup inside that? You could use foam for fast speeds and many passes to get a good surface -- then finish it off with gelcoat or similar product for a "perfect finish." You could then use the foam as a trial or "proof mould"- but it might not hold up to many mouldings.
BUT you could take a resin and fiberglass mold off the first product, -- gelcoat it, and then multi layer fiberglass, and supporting bits and resin for a bullet proof mould.
This approach adds a step or two -- but gives you a "proof" model at low entry cost. You can then tweak it" to get the final parts to mold from.
[NOTE:You asked for plug materials - and the same approach could be done using foam for the plug.]
Best of luck - Welcome aboard, and
:cheers: Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.