If you don't want to spend any money you can just lean the door out and put some HD aluminum foil in the gap against the door then shut it and trim the foil about 2" or 3" around the door. Just take the foil sticking out and use it to fill in the gap gently working your way around the door. I had to do this when mine was new for a while and it works well. Like others say if you want to buy a fix just get the Cajun Bandit door.
The stock door works fine for me though, I bent it to fit the curvature of the middle part and although it still leaks a little temperatures are pretty stable.
22.5 WSM and a couple charcoal grills.