I have a 2'x2' box that stretchers the width of the truck bed and sits on the rails up against the rear window. Its as close to cab dimensions as possible to maintain aerodynamics on the highway. There is a latched door on each side so the dog can be accessed from either side and there is a sliding door on the back so the dogs can load and unload themselves into the truck bed. A couple plexiglass panels allow me to see into the box from my rearview mirror and see all the way through the box to obstacles behind me. I stole this design from dog musher boxes that are common up here in AK.