OP- nice mods!
I used high temp silicone to seal my doors. Wiped it down on the flange area good with brake clean and then followed up with non ammonia window cleaner. Then I cut the tip fine on the silicone and used it to seal the edge gaps where the main body meets the door flange. Did the old wipe around with the finger deal to make sure it was pushed in there well. Then ran a bead around the door opening on the flange and placed plastic wrap I'd sprayed with PAM over. Laid it so that it only rested on the outer edge of the cook chamber and the top of the silicone bead. Did not want to get PAM on the flange. Then carefully closed the door and left it.
Now just have one tiny wisp on one edge. I'll either re silicone or perhaps consider latches to suck the door down even tighter.
One tip, although you need to be careful doing this, I found worked nicely to start with on my firebox door was to hammer and dolly the flange so they fit well. Was not welded on well at all on my pit. Had gaps between the pit and the flange. Did not take a lot of work to get the flange gaps between the box and it much tighter.
Question on the chimney. Did you just pic up a small piece of 3.5 od exhaust pipe from an auto parts store? If so, main chain like autozone, oreilly etc.?
Not to highjack as posted about this with my own post, but have you experienced any issues with the size of the inlet port? I ask as the port looks to be approx. 7.5 sq inches and according to the calculator it needs more like 15. At least on the 20 x 40 cook chamber one sold at Lowes.