I'm in a dry climate so not worried about rain. That coroplast looks like the ticket. Some glue tests show a hot glue gun with special yellow sticks (3M hot glue designed for Olefin plastics) may be the best adhesive here >
https://www.christinedemerchant.com/...glue-test.html
I like the looks of this. As I have 17 hard case cells to hold in place, I leaning towards building a box of some sorts and then some way of holding them down too. Maybe even hinging a lid of this same material? That would prevent anything metal from dropping into the pack. Hmm