Well, with all of the complaints about WSM doors of late, I think there is an issue with the latch maybe.
I have the older style WSMs with the round knob. I have had success by modifying the latch assembly with washers so the door fits tighter. My doors all leak, but not to the extent of having temp issues.
You should not need to gasket a WSM door if you get it to fit right.
Best of luck figuring it out.
My Boys: Joseph (EI); Andrew (EE); Daniel (EE)
My Toys: 10 WSMs (B, C, E, DH, DA, EZ, DZ, EO, DU, DR); 5 - 22.5" Bar-B-Q-Kettles; 2 - 18.5" Bar-B-Q-Kettle; 3 WGAs (EZ, DE, N); 2 SJPs ( DR, DR); 2 Smokey Joes (A, K); Genesis Silver C (DU); Brinkman Cimmaron.
"For when democracy becomes tyranny, those of us with rifles still get to vote"