Next layer, 1/2 inch plywood.
Use the already cut foam as a template. I'm cutting the plywood so that it's seam and the foam's is offset from each other.

I'm laying the plywood crosswise as opposed to lengthwise so there won't be a seam running the length of the van.

The downside to running the plywood this way is the metal edges that stick out. To get the plywood past them you have to trim off part of the end. Then just lay the trimmed piece in place.

One tip. The drivers side rear corner has a irregular shape. I found this method for creating a template handy. Cardboard strips. Place them one at a time against the wall and tape it down to the foam template (who's edge didn't match the wall to well when I first cut it).

With that template you'll be able to make a good cut.
