3/4″ sanded birch plywood. The shelf gets cut first. Want to get to underneath the shelf once in a great while. Stick to prop up? Hooks from the ceiling?  It is 21″ wide, 56.5″ long. The bed is 52.5″ long, 25″ wide. Here are some concerns: what will keep items under bed/shelf from sliding out on turns? How troublesome to retrieve items from under shelf/bed if there is a slide-stop?  Working on a way to work on a computer inside …?  “Table” across from foot of bed? Or would it be better to have a skinny shelf on the passenger side?

2×4’s for support vertically. 1X2’s horizontal support.

The tailgate gets a layer of plywood instead of the existing slippery plastic stuff . I like the idea of that tailgate mod. Kind of like the picture below.

These ideas have been updated here