Polystyrene Reinforcement?

I've been doing a little bit of searching throughout the forum but with little luck on the matter. I've just casted two lids from my scratchbuild, so my question is this: Does anyone have their own recommendation(s) when it comes to strengthening their buckets? Would applying fiberglass (if it can adhere) or even Bondo on the inside help any? The main things I'm concerned with are possible warping/losing shape or cracking the paint if the lid is flexed. Any suggestions?

Fiberglass and resin will be your best bet. Most of the time that is what I and others have used to reinforce helmets and other armor. Bondo might work, but you would run the risk of it popping off and it would be heavier than a good fiberglass layer.

Fiberglass was the original plan, it's just that I got the impression here and there that it wouldn't bond well with plastic. In any case, if it is still a viable choice then I'll stick with it. Thanks!