i think the epoxy route would be the way to go if you were going to be building a production style plug but if its a one shot deal why not save yourself the hassle of the sanding and expense and go quick an durty with somthing that works down fast. i went this route with a tank cover for my 77 bronco one time. took me a day to fair out the old beat up rusted one with plaster, PVA'd it and pull a quick mold . knocked out a new part the next day. the only drawback is it leaves you with kind of a rough part that need alittle finishing after its out of the mold. 6 of one half dozen of the other i guess. no reason to re-invent the wheel here.

PVA is polyvinylalchohol. sort of a sprayable water based vinyl barrier coat. you can get it a west marine or boat US. i guess i suggest it if you havent worked with fiberglass molds before. its an almost gaurenteed release. you can cook a part in a mold real easy if your not careful.