Instead of ply-wood straps in channels I put foam-straps and then covered with 1/2" birch play-wood. Everything attached with a liqeed-nails.
Surly as additional attachement use cargo-straps "D" bolts for a screw in 1/2" ply-wood panels
You don't need all that foam (unless you a planning an ice delivery business on the side). GaryBIS (PM him) is a poster here and developed an insulation calculator. I ended up with 1" of polyisocyanurate as enough using it. I'd get a foam cutter and go back to the ribs and then shave everything flush with them. Spraying that in is expensive so it should be a good thing!
Post your solution, Please.