Had a bit of a measure up tonight, and the dimensions for a piece of ply to suit are pretty friendly.
If I make some drawers for the back, or use boxes of the right height to match the folded down rear seats then a half sheet of ply will completely fill the gap between the boxes and the front seats (once its trimmed).
If I then cut and hinge the ply lengthwise, when its folded in half it will fit nicely across on top of the boxes/drawers for travelling.
Possibly this is the first time ever that the size of material I need is JUST SMALLER than a standard sheet size. Usually I seem to need 2 sheets of anything as the size I want is about 50mm bigger than standard!!
Now - to find a cheap piece of that lightweight stiff Asian ply - the stuff that almost feels like it has a balsa core.

