Do you really need anything more than a poly tarp?
The other day I broke up a bunch of screen doors, and took the metal scrap to the recycling station. For that I lowered the front passenger seat back lining it up with my cargo plaform. Then layed a poly tarp over the whole length. The ends of long pieces (about 7') were stuck into a duffle, and the bundle tied together.
My cargo platform if functionally much like the rear seats in their flat position.
Raising the rear seats gives a rather clean 4 x 6 box. A tarp could be fitting in there, bathtub fashion, protecting the interior from scratches and debris. If needed, bungees could secure edges to the headrests and grab handles. With some cargo I can see the point to adding some stiffer panels (on the cargo side of the tarp) to add further protection. But if the cargo could shift enough during travel to harm the plastic on the other side of the tarp, it could also cause serious damage to the driver during a crash or rollover. In which case I'd worry more about securing the cargo with a tarp or net on top to keep it from shifting.
Insulation panels, bubble pack, foam etc would work just as well as plywood to protect the interior.