I wasn�t quite sure how to answer your question, so I asked my dad and here was what he said. Most late model pick ups have notches on the inside of the bed that serve as a resting point to have three 2X4's laid across the bed at the height of the fender wells. This allows for the driver to lay a 4 X 8 sheet of plywood across the bed to make a sleeping platform. Camping gear, clothes, etc. can be stowed below the sleeping platform as well as carpet with padding to insulate the bottom of the truck bed. As far as insulating the shell itself, the simplest method would be to buy bead board or foam board at Home Depot. It comes in 4 X 8 sheets and can be glued to the inside of the shell after cutting it to fit. Care must be taken to use glue that will not melt the styrofoam insulation. A preferred method would be to frame out the inside of the shell with 2 X 2's and use batt insulation and then cover it with paneling. This involves some skilled carpentry work and is not for the unskilled.
Most spray foam insulation is quite toxic, so if you are looking into that method, make sure to spend some extra money and purchase only non-toxic stuff; that is way too small of a space for you to be breathing in anything bad for you.