Dyno,
You didn't state what your "plug" was made from. Styrofoam is easy to work with but melts when in contact with polyester resin. You might be able to wax it (buildup of wax would have to be thick). You could cover the plug with wax paper as suggested, or simply cover it with masking tape and then wax the tape of spray it with a relaese agent. If the plug is solid and will be pulled out make sure there there is a taper from front to back so it will release. Where the fairing will attach to the body grind off paint and primer to bare fiberglass or metal. Hope this is not to simplistic.
I have done a lot of fiberglass work ( making a one piece '57 Corvette 'glass body back in the '70's, how stupid was that!). To a lot of pieces on my race car, in fact I'm doing some glass work today on a air intake duct. Off subject.
Another way is to use 1/16" fiberglass sheet (available from most plastic suppliers) in 4' X4' or 4' X8' sheets. it's easy to cut and is ideal for non-contoured parts. Fit up the pieces to make the inner dimensions you need and then glass over it, sand and paint.
High density foam is easy to form and hollow out. It resists polyester resin and pieces can be glued together using Bondo. It can easily be shaped using a "cheese grater" type body file. It's rather expensive and some suppliers require full sheet purchases.
I made a handy little conversion chart for resin required per sq. in. of F/G mat so I don't waste to mush resin. You want to mix a little more than less resin. After laying up the Mat do a rough sand of the surface and use a wet resin coat utilizing F/G cloth as the final layer this makes a smoother surface for paint prep.
Hope this helps.
Tom
(time to get scratchy, then cold shower time)