I don't know if it still is the norm but, at one point in time, many manufacturers were making their canopy plugs from Bondo. You might be able to pull a mold with thick styrene or ABS and fill it with Bondo. I'm just not sure if the plastic will take the heat generated during the curing process. If it worked you would then have a solid (or hollow, if you desire) Bondo plug which could be sanded to a near perfect finish. I suppose you could pour it into a plaster mold also, if the plastic doesn't work. I've seen several very nice (and very large) canopy plugs made this way and the quality of the parts pulled over them was outstanding. The one that comes first to mind is the Rick Lewis designed Skyraider with which several well known modelers competed some years back.

PS- Bondo is a polyester product and can be thinned down with polyester resin for pouring purposes.