Building this for a guy here locally for his deer lease rig. It is aimed at being used to move people more than to hunt out of, which is why we decided to put it down lower in the truck to get them out of the wind. Materials being used are 2x2 1/8th wall angle, 2.5" 1/4" flat bar, 1.5" 16 gauge square tube, and both 1/2" flat and 3/4" raised expanded metal.
Materials to mount the rack to the bed. It has to be removable because to get to the motor you have to take out a piece at the front of the bed.
I welded the washers and nuts onto the bottom flat bar to make it a little easier to get off and on:
Tacked up the main part of the seat and part of the footrests: