The walls of box bodies are now commonly built out of GRP which is glass reinforced plastic bonded to structural plyboard. It has a very good strength to weight ratio with the benefit of being able to screw things to it on the inside such as lights, racks shelves etc. You can also get extruded or moulded corners for it so you don't have to worry about trying to build a frame work, you just cut corner lengths and bolt the to the wood.
Aluminium Corner Panels
As you can see in the above link you can build a frame out of 8 lengths, the board slides in the edge and you use a coach bolt every say 6-12 inches depending on the length. GRP also have the advantage of coming in massive sizes, its not uncommon to get panels 45x16ft in one piece!
Another option is to use corrugated aluminium sheets (not the big stuff used on roofs) each ridge may only be around 1/8 or 1/4", the ridges build in strength. You can then build a frame out aluminium 'top hat' section or box section and rivet the aluminium sheets to it with a small overlap between sheets.
There probably isn't much weight difference due to the extra frame work involved in the aluminium option. So there's pros and cons.
GRP panels and corner posts:
Very Easy to build
Easy to attach things to
Difficult to transport the panels after you buy them
If the outer layer of GRP gets perforated the wood can rot if not sealed
Aluminium frame and sheets:
Easy to get hold of everything needed
Will not rot
Difficult to attach things such as shelves and holding points to, as you can only attach things to the cross members and you can use coach(headless) bolts.
Difficult to weld
You need hundreds or thousands or rivets which each need a hole drilling.