Found a CHEAP solution that I'm very happy with! This $21 restaurant shelf (link -> 12"x54" shelf) fits perfectly in the rear and rests just atop the cargo tie down points on the rear trim. Secured with heavy duty zip ties and put some thick adhesive rubber "feet" on the corners to keep it quiet. It should hold all kinds of extra cargo and has a weight limit only limited by the strength of the plastic rear trim.

 

Inspires me to take a measurement and head to the nearest big box lumber store. I seem to remember they have a whole shelving aisle with pre-made metal wire shelving in a wide variety of options. Thanks!
Now to figure out what to do with the left over shelves...

 

Exactly! And like the $21 shelf linked in my post, you're looking for something that's in the 54-56" width range and I would say no wider than 16". No reason to pay $200-300 for one online...