Open the smoker's door widely enough to access the inside panel and the smoker compartment's outside edge, which abuts the door.
Apply a self-adhesive, high-heat rubber gasket to the inside edge of the door where it presses against the smoker's shell. Such gaskets are available in a range of temperature ratings; use a gasket that matches your smoker's heat requirements.
If the gasket doesn't fit the space, cut it to fit before you peel off its backing.
Apply a high-heat silicone sealer for a custom fit. Place a bead of the silicone sealer on the edge of the door where it closes against the smoke chamber.
Cut a strip of wax paper that will fit atop the silicone bead. Lay the wax paper on the silicone, and close the door so the seal spreads to the proper shape.
Open the smoker's door, and allow the silicone to dry according to its manufacturer's instructions.
Remove the wax paper by lifting it away from the silicone. Use a razor blade to cut the silicone bead to the correct shape.