Bed and shelves in truck bed

The bed, on the left, is 25″ across. I tested this out, and it works. As long as I have room at the head to prop myself up and read it will suffice. The bed and opposite shelf will be 17″ high. That 17″ height is necessary for the height of the water containers, the propane tank, the poop bucket and the access to the house battery. The shelf opposite the bed will be 15″ wide. That will handle the milk crates underneath. This configuration will leave me a 16″ path in the middle.
I still haven’t figured out the kind of plywood for the bed and shelves. Marine?  Birch? Should I have a plywood sheet under the legs to hold everything together? Can I get away with 2’X2″ for support or 2″x4″? Should I have corner braces or just screw everything together?

I will figure this all out as soon as I can start assembling. Still don’t have a truck. These numbers are for the imaginary Toyota Tacoma.