This is how I decided to mount the filler door for the rear mounted tank. I used a magnetic filler door from Yogi's
here. When grafting it in I wanted to not just have big flat spot in the middle of the ridges in the bed, so I added a few angled pieces to mimic how the ridges terminate on the bed originally. I know the one in front of the door toward the cab will trap water in the valley but I am not worried about it. I have a Raptor bed liner kit that will be going on once I get this all cleaned up. Next I will be using my CNC machine to make a template for the bed ridges so I can hammer and wood dolly out a few more bed dings before adding any more body filler. Here are few pictures of a wood dolly someone else posted a few years back, and a few pictures of how the filler door is going.