Just welded one of my doors up.
Given that mine rusted from the outside I just cut a strip of 20 gauge and just stuffed it inside the door tacked it in place and just welded the holes up with the MIG.
I plan on fixing my apron panels (bed to running board) in this manner as the metal gauge is too thin to weld up properly. However, I decided against that method for the doors as the backing metal is a place for moisture to collect and cause rusting between the two pieces of metal. Especially in a door that sees a lot of water. On the aprons, I can seam seal perfectly due to unlimited access.
