In the episode "Daisy's Close Call" (S6 E19) Cooter is seen using a tool box from the rig. I can't see it in any other episode, so, being Hollywood, it probably wasn't there at any other time, not being a true "working" rig. Still, I thought it would be a great, even necessary, addition.
I estimated the size at about 20" x 8" (when he carries it to the truck it is indeed fairly slender). From some angles the grime from being handled by a mechanic is visible. Appears to have a silver or light colored handle.
I scratchbuilt it in an evening, primed it and painted it the next day.
The same day I scratchbuilt a crowbar, and added a hammer and a wrench that came with the two junkers from which this project began. The wrench was finished in Alclad stainless steel, then grime strategically added. All the tools were made to appear worn and heavily used. Finally all this was added to the wrecker bed, including an extra length of chain visible in the episode "Happy Birthday General Lee".