Excellent job, JT! Very impressive.
You didn't say where you had found your foam, but around here, there are what are generically called "foam and fabric outlets", which my Mom haunts frequently for her sewing addiction. They typically stock all kinds of fabrics and materials for sewing, in addition to lots of different types of foam. Usually they'll have a cut-off bin where you can find scraps really cheaply. If you have any places like that around where you live, I'm sure they'd have some kind of useable foam for this and future boxes (after seeing this project, surely you'll be making more ;~).
The other source that you might check, if you haven't already, is camping and outdoor stores in the sleeping bag area.
Good luck in your quest for foam.
Bryan