You could make an arrangment like this:
The top of the rod has a 3/8" bolt welded to it (threaded end up). There is a spring, flat washer, and adjusting nut put on the bolt before it is placed through the flange on the smoke stack. The top nut is welded to the flap. The bolt is threaded through far enough it can be double-nutted to make sure the flap will turn with the handle. You can then adjust spring tension to ensure the proper seal between the flap and the stack.
There are several ways to accomplish what you want. This one seemed to work well for us.
Our other smoker has a damper inside the stack like the ones in the stack of a wood stove. Don't have a pic handy of that.
Hope this helps.