Here are a few Photos of a Plug I made for Smoke tanks on A Fly Eagle F-15.
First photo plug in place in Fuse.
Second photo first layer of fiberglass cloth with Epoxy resin.
Third photo Heavy code of Bondo after fiberglass to make smooth. Most of which was sanded off
Fourth photo first half of mold being laid up.
Fifth photo first half of plug made.
Final photo finished Mold. The lines in the orange is because I could not find my Dump gun so I had to brush on the gel coat.
Its not a hard process but it does take some time sanding and getting the plug smooth. The finished part will only be as good as the plug.
Hope this helps.