[Xist]

I don't know why I thought I needed to cut the insulation into the pieces. I cut each one in half, slid it from the middle to the edge, shoved a 2×4 between the insulation and the 4×4, slid in the other piece, lined it up, and then shoved in a spacer on that side.

I needed to plane a bunch of scrap wood because somehow a 2×4 was 1/4" too thick on one side.

The other side doesn't actually seem to be touching the 4×4!

I only need to cut four more pieces, which I will cut in half before installing, but I think I will seal everything with Great Stuff first, and then reuse the scrap wood.
I knew the batteries in my crazy bright head lamp wouldn't last forever, but I used them until they died, swapped them with ones fresh from the charger, and then needed to use a far weaker head lamp for at least an hour.