Ply is, as long as you want to stick with anything wood, far superior for the purpose to anything else. Nothing else will match its flexibility, durability and fastening properties. Of course you do need to beware of the downsides to it as well, and not just leave screws or nails to keep it together in on the edges. Chances are it will not hold up in the back of a van. Some kind of frame, kleets, brackets, dados or what you may prefer in conjunction with both glue and mechanical fasteners should give you the desired result.
Of course you can always edgeband and so forth to your liking with something else. In my former van i put on softwood strips on the uprights, as not to damage fastened goods on the cargo floor from rubbing against the shelves, as well as having a tiepoint. Endless possibilities really.
