Making fiberglass cowl

refer to = scroll down to = Mold, Cast & Vacuum Form items yourself FAQ inc pictorials (see also Fibreglass etc)
Moulding & Casting Mould Making . Model Car Tech
and related sections for helpful advice.
If using plaster, then either use the Fibrous type used by Plasterers to fill gaps in interior wallboards - the fibre holds it together. Alternatively use the molding plaster and plaster cloth from the model railway section of your LHS (or A&E at your local hospital if you can find a good excuse to visit the nurses.). Drying time depends on how thick the paste was, and weather conditions - 1 day to a couple of weeks. Place beside a hot water cylinder to dry slowly.
regards Alan T. Alan's Hobby, Model & RC Web Links

It may take a week to ten days for the piece not to feel "cool" to the touch. Avoid the temptation to microwave the piece or put it in the oven at low temperature. It's a bad idea.
Once the piece is thoroughly dry, you might try coating it with polyurethane varnish. Use a 50-50 varnish/thinner for the first two coats to get deeper penetration. Might even use 75 thinner 25 varnish. There is also a water based polyurethane that might do just as well, but I've not used it in this application and can't speak to what the water might do to the plaster.
Another alternative is to coat the piece with thinned 30 minute epoxy. Try thinning with Goof-Off or 91% rubbing alcohol (although now that I think about it, the 91% isopropanol may not work. Try a little and see)
You want an almost water-thin viscosity to allow the sealer to soak into the plaster as much as possible.