Just slightly over bend the top of the door for a good fit. Start at one top corner and start bending gently in towards the other top corner holding the top edge between your palms and fingers. Move across the top edge bending little by little every inch or so until you get to the other top corner. Just don't bend it so much at any one spot that it starts to fold.
When you're done the door is actually bent to a tighter radius than the smoker. When you install the door the aluminum will flex enough that you can still latch the handle and if you were careful enough, the door will fit extremely tight. The door actually has to mold to the smoker to allow the handle to come in far enough to latch.
Good luck and be gentle. You can always bend it tighter if you have to.