Hey Max,
Might be a bit late now, but for the next order, if you've still got the laminator, then flood coat a sheet or two of foam board. Makes a quick and easy way to beuild the walls of a box, cut 1-2" strips of self adhesive board, and stack them up.. Then as said, more and more rigid outer layers. I have a corner where I pile old bits of stuff for packaging - 4'x10" bits of foam board etc, too big to chuck, too small to use until you need to - used to drive my assistants mad. But makes good recycling.
I also have clients who will ship back packaging in advance of new orders.