All the supplies I used for this posting were already on hand.
After I mounted the first insulated fiberboard panel I noticed the empty space behind it.
From left to right in the picture below is the inside of the van, fiberboard, extruded foam glued to the fiberboard, empty space where the rear windows would have been mounted, Reflectix insulation on van wall.
This gap varies from 1-1/2 to 2 inches.
That's the top half of the wall. The lower half also has voids but not as large.
That's too much space to ignore so I'll use pink fiberglass to fill it. Since fiberglass doesn't do it's job if you compress it too much I'm going to make a custom fiberglass panel. This is a Proof of Concept since I haven't done this before.
Start by covering the area to insulate with a plastic sheet. You can use a magic marker to draw directly on the plastic where you need the insulation.
Remove the plastic sheet and spread out on the floor.
I'm working in the basement and it's cold this time of year so I've spread out foam to keep the plastic off the floor.
Use a paint brush and cover the area you previously marked out with contact cement.
The rolls of fiberglass I had on hand were 3-1/2" thick. So I wanted about half that thickness for the top half. If you're careful you can peel apart the insulation keeping the thickness roughly where you need it. When you need to trim a piece don't try and pull it apart. A really big pair of scissors comes in handy for this.
Put the fiberglass down on the prepared plastic sheet.
Continue with the lower half. Notice big scissors.
For the lower half of the panel I only wanted around a inch thick layer of insulation but had a problem keeping it consistent as I peeled. I ended up with a patch that was too thin. Just put some more contact cement on the fiberglass and pile a little more pink stuff on top.