Check Amazon for the term Coroplast.
Corrugated Plastic. That's what came on the bottom of mine. It's convenient but can be costly.
But it allows me to open a section and then can close it up again with 2X3 panels using #14 self tapping screws and gorilla tape for a water tight close.
Here's a link from amazon for a roll, but you can also buy this in different size "signage" sheets.
Also available at Lowes and HD in the signage section, but even more expensive.