Insulation from or for reality?

Finally I get a job done. The gas covers are finished and in place, which then means that I can finish the wall insulation-or at least try...

Riveted on and body sealed up the yin-yang

Some of this stuff was finicky hand cutting. But when it's all done such a sense of accomplishment!

As I was working away I decided that perhaps I should get the interior window wells in place, or at the least figure out which one goes to which window.

But....as I paired the interior window skins I came across an "issue"

My plan was always that I was to place a backing board into what appeared to be a preset recess for this very purpose. What I didn't remember or realize was the position of the board relative to the space.

As you can see the slot sites well within the insulation space in the wall. The space that would be available behind the board would only be ...1/2"? Being that I want 1 1/2" EPV foam board... this just will not work. I have to come up with another solution.