I'm a bit late on this one but since I just did this I thought I could help.
I found plaster a bit weak on it's own but it can work great. The best way I found to make another part like a canopy with plaster is to make a frame to hold the canopy, pour in the plaster, let it dry for a week or two, spray it with fillable/sandable primer from auto parts store, fill holes or dings with body putty, sand smooth, pull a .015 sheet over the plaster mold, trim the excess off, glue the edges down, glue 3/4" wood under it to raise it up, sand with 400, now you can pull as many parts as you want. The second pull will not stick to the first. Blow some air under the second pull and bingo! You have another part. Oh yeah, buy extra plastic!

I also found that to make other parts from scratch, balsa is the easiest. Carve, shape & sand the balsa, fiberglass it, fill and sand.

It's a lot of fun when you start getting good parts. Good luck!