The rear panel can be cut to size to be butt welded to the side deck panels. When you have overlapping panels that need to be cut for butt welding, a quick way to get exact fit is to first lay 2" tape along the edge of the panel that will be below
Then position the next panel overlapping and in the exact position you want. Lay another strip of 2" tape on the top panel and line up the upper edge of the tape with the upper edge of the first strip of tape.
Mark your cut line on the lower edge of the tape. Cut exactly on that line and you will have a perfect fit.