In addition, I decided to raise the bed 4" which will be just enough space to fit the SUP's storage bag. I'm also taking the opportunity to remove the old framing for the L-couch that we never used. Having a big open space will allow for plenty of storage and possibly bringing the fresh water tank back inside the truck as a future project.
I first removed everything in the back:
I plugged the hole of the old dinette (a wooden plug and construction adhesive) and then gave everything a good sanding before applying DIY bedliner:
DIY bedliner is not the best but it should be fine for this application. I used the version with accelerant based on reviews and was pretty satisfied with the result. It should help prevent things from sliding around, protect the fiberglass and possibly, with a bit of luck, prevent a little bit of road noise.
I then started building the frame using 8020.
The weight to strength ratio is pretty high! Once I put plywood on top and below it should be plenty strong! I'm also going to place 1" foam (the same size as the extrusions) to insulate that compartment from the cabin.