Big Mista or Big Brother Smoker posted once about using flat piece of plywood for a lid. Worked for him without any crazy rub down. Maybe someone can find it.
I find it slightly odd that you have to keep all three bottom vents open to sustain temps. That is odd. In this case, I don't worry too much about a slightly leaky lid. Usually the leaks are more a problem with keeping temps down. It might have more to do with not lighting enough charcoal at the start.
Proudly Smoking on a UDS, PBC &
Accidental Winner of the 2012 "Saucy" Throwdown.