What I did was rough up the back of the plastic REALLY WELL (after forming it) with some 40-80 grit sand paper, then fibreglass the back. It worked really well and made the plastic far stronger and thicker.
:cheers
A'den
EDIT: Forgot- I trimmed the edges of the fibreglass with a razor knife after I had all my layers on, then put one more coat of resin over it to seal it down.
Thanks, A'den. I'm gonna try it out. I definitely need more hands-on time with fiberglass & resin anyways for my buckets, so I think this would be good practice.