I used one of these
that I put inside the flat lid. It snugged up tight against the inside of the bulge of the flat lid that sealed over the lip of the barrel. Then I just used a bead of JB weld to adhere it and seal it. Worked better ''n any other lid I've ever made. Sorry, the pics I took of the finished smoker (which I gave as a Christmas present to a friend) got deleted from the camera in order to take pics of the kids opening their christmas morning goodies.
And let's just get this out of the way now: YES it is galvanized. The temperatures that I cook at are no where near hot enough to vaporize the zinc and I figure if you can abate asbestos with paint, you can abate zinc with a good cast-iron style seasoning.