2nd-hand Prius w/ reciever hitch for platform or trailer >> long wheelbase/tandem axle
pickup conversion >> electric dump bed. Pay as you go from business profits.
I was in a landscaping cooperative in 19-mumble-mumble. We had a 1954 International with a 3/4 ton flatbed with side racks and a hydraulic dump. It's very important because when you are hauling bulk materials it halves the work.
It could be made from an aluminum trailer box of the appropriate width.Solar panel side racks hinged on the top could flip up so the one on the off side could catch light as well.