The IBC does infact have Polarity protection. We had a competitor running at Robowars 2003, KO, 4 x 12v drills on 24v with a possibility of drawing over 200amps on a moderately large drain and it only got the IBC warm, that was with out a fan on the heat sink, and Tim was making KO push a fair bit, I know, I was able to get a beautiful view of two sides of the arena .

On both of our IBC that my team owns, we have small 40mm PC fans on them to have that extra bit of protection, expecially when running T2M with its 340watt winch motors (When you see your team mate who is around 80kg sitting on top of a feather weight doing over 5MPH in the pits doing a wheel stand on your feather weight, youll under stand why we have a fan on the IBC )

Brett was running BullDoze, a heavyweight on an IBC using wheelchair motors at 24v and it was getting fairly warm, but it with stood the punishment.

Ed, if your going to make up a finned heat sink, let me know on the prices you can get them made for, you might be able to sell them as IBC Upgrade part with fan attached