Mike,
My Chorus I's came in factory boxes with a cardboard-backed foam "tray" for the top & bottom like the ones mentioned, these were only about 4 inches deep and about 3/4 inch thick. The strength of the box came from 4 V-shaped extremely dense cardboard "corners" that stood inside the box. By V-shaped I mean that they were a piece only about 5 or 6 inches wide (maybe 1/8" thick), bent 90 degrees... hard to explain, but really just a reinforcement of the boxes corners (or edges, running vertically.) I tried to keep my original boxes, but after 12 years & almost as many moves they didn't make it... but I never saw the need for 2" of foam all the way around! Of coarse if you tried to reproduce what I just described someone would surely skewer the box & prove me wrong! I'm only an hour or so away from you, I'd give you my old boxes if they were still any good!