I couldn't get my Weber lid to fit too good either at first. Lots of air leaks. Held temps around 250-275 okay with just a half ball valve open so i installed two latch clamps to hold the lid down a bit tighter and sealedup the air leaks pretty good. Now she chugs along at 225-235 for hours on end no problem. Also good to keep FIL from opening the lid to check things out for no reason!!! last time he was here he didn't get the lid on correctly and I had 300+ temps on my hands...may have to install some kind of lock to keep the lookie-loos out ofthe drum.
2004 Weber Performer GREEN
Orange Super-Duper Fast
'08 Weber Genesis Gasser
SJG Tamale Pot Mini Smoker (SJG was a rescue)
and a Pyro FIL