Faced with similar difficulties my 'solution' for my 8x10 kit was a rolling tool box that is sturdy enough for me to sit on, and a 2nd tray-type box that attaches to the top of the 1st. The 1st has the camera, film holders, darkcloth etc, the tray has lenses. So I can roll up, put the tray on the ground, set up the camera and tripod, then sit on the toolbox if I need to rummage through the lens tray, or rest my legs and back, or I can even shoot from a sitting position if the perspective warrants it. Or I can put the tray back onto the box once I have set up the camera.

The motivation was more not having to carry 50 lbs of camera gear on my back, and having something to sit on, but this was the only affordable solution I was able to find. I considered the folding rolling shopping cart things I see people walking to the grocery store with, but with that I would have to pack a stool as well, and I would then have several camera bags and the like haphazardly piled around me.