A heating device and method for use in heating a canister containing a reductant, such as ammonia for use in an after-treatment device in an exhaust system, is disclosed. Two screws are provided to be threaded into the inserts through the post channels in mounting the posts and bar to the door. The overlapping flap has a plurality of small lap portions and a plurality of large lap portions for overlapping the rear surface of the peripheral edge of the opening to a small degree and a large degree.