That's a really good post, Tommy. I would also add that it might make sense to "back-step" weld. If the box has to be water tight with the lap joint situation you propose, then once it is all tacked up with your chill bars in place, weld 1 inch of metal (fusion only) and then stop. Weld 1 inch, ending the bead on the start of your first 1 inch of weld. Follow this process until you have one side of your box completed. This will help with distortion as it will let you start on a "colder" area as opposed to moving the heat all at once which adds to warping.