Here are the steps:
This is the 18"24" campaign yard sign (I have already made the cuts necessary). It is made of corrugated plastic.
After measuring for the panel size I need to store (9"x12" for this one) I laid out the fold lines with pen and used a razor blade to carefully cut these channels to allow for folding.
Testing the folds:
Guides are spaced to store a single panel on each end and two back-to-back in the middle.
The box assembled. The two side/edge pieces are taped into place with strong duct tape - Gorilla Tape in this case:
I stuck on three hook and loop fasteners to the top flap will close securely:
Here's what it looks like with the panels loaded: