Flatbed update; CM did come back and said they would build an RD to our specs for an additional fee, more to follow.
Didn’t forget the back of the cab and don’t know if I’d recommend this route but it is what we did (thinking bulk carpet would have been easier). Ordered a gas tank cover hoping it would be enough to cover with some minor trimming. Mrs. 7dee2 pulled the seams apart so it would lay flat and we found plenty of material height wise but it is kind of close width wise after trimming it. She removed all the plastic tank clips and came to the rescue by added some of the extra material to the sides. Test fit went well, this is going to work with the seams just inside of the speaker mounts.