I may be better off just replacing my FB completely,
It would be the best solution, ....going with one about 25% larger than recomended, double plating the top, and going taller to give the fire more room to breath and create a better draft would turn that into one really nice smoker.
Good thing is, you already have something to cook on, so you can take your time and really build a nice one, then just swap them out when its ready. The increased draft would probably compensate for the couple sq. inches your short on your opening, so you probably dont need to raise the reverse flow plate, just clean up the opening the best you can so you get every inch out of it.
Its a nice looking smoker!