RE: New twin design project
If you made the plug a little longer, say 1/2" or more than you needed, you wouldn't have to worry about any glass or resin over laying onto the backside of the plug. Room to spare so to speak. You'll need room to trim and sq. it up. I'm thinking that the way it is you'll almost have to go past the end of your plug which means that you'll have excess on the backside of the plug that'll have to be trimmed off before you can remove it.
If you were to make a mold from your plug, then you could make your 2 cowls with the mold and the exterior of the cowls would be a lot smoother to start with.
I'd check with a body shop or call a fiberglass business to see what they recommend for the release agent. It's been 30 yrs. since I did that type of work and I'm afraid I've forgotten a thing or 2. Maybe 3.
Slowjohn