Check a few posts down in the Fabrication forum... you'll find my thread on finding a good plastic adhesive:
best plastic adhesive/filler?
I've gone and used the Fusor stuff, and it seems to work fairly well. It's messy, but it cures fast and firm, and sands to a smooth finish.
Someone else recommended a product called "Rigid Filler" made by Dominion. It appears to do a very good job as well. Check out it being used here (try the Misc Work column): http://hzemall.com