I don't have a lid for my brewpot. There is no need, and boiling with the lid on causes more problems anyway.
If you're having trouble reaching a boil, you might have to use a lid to bring the wort to boil, but after that you shouldn't use a lid. I don't use a lid while chilling either, since the steam should be released and not allowed to condensed on the lid and drip back into the boilpot. I boil indoors, though, so don't have stray leaves and bumblebees around my chilling wort, either!
Broken Leg Brewery
Giving beer a leg to stand on since 2006