I haul lots of crap so I removed the panels and built a plywood box for the back.It keeps the debris from wandering and provides some storage between the plywood and the outer body.As it is a cargo van,I had no fuse box to deal with but there was some thingamajig rear of the passenger sliding door that I had to box in.
The handiest spots are some cubbies just inside the sliding doors for binders,bungies and often used tools.