Thanks for the thought on the sliding door storage. I managed to slide a couple of longer things into it already but will have a go with a proper solution another time - probably just with a piece of plywood.
Storage by sliding door (all in centimeters)
Outer vertical panel: 40 high x 20 wide
Inner vertical panel: 26 high (25 probably better) x 31 wide
Horizontal panels x 2: 31 wide at inner end, 20 wide at outer end, 20 deep.
Shelf fronts x 2: 20 wide x 4 high
The horizontal panels need a curve cutting out to fit the curve of the corner with the air vent - used a contour gauge and was fairly accurate.
Beware that the table for the sliding door will then have a smaller space to be slotted into as the shelf encroaches into the room used to get it into position, but it seems ok.
I am also considering trimming the top outer lip to reduce the risk of someone carltchi g it as they get it of the van.
The remainder of the ACM was used to make 3 sides for some storage I have in mind on the left of the rear tailgate area. I recently made something for this in cardboard reinforced with old Tupperware box sections but I think the ACM will be better.
48 high x 20wide x 5.5 deep
All the sections were initially templated in cardboard to find the best combination for cutting the ACM but with the measurements above it should be possible to going straight to drawing out in the ACM protection film and then cutting.