Corrugated poly looks like a generic CoroPlast. I don't exactly remember the numbers now, but I believe the stuff I used for planes was 6 mil.
They also had sheets of 2 mil (way too flimsy for the OP's job)...and something like 10 or 12 mil. The thicker stuff was stronger and somewhat heavier, but probably more expensive.
The 6 mil came is 4 x 8 sheets and they also had 4 x 12 sheets, but not at their plant in Houston, only in Austin, TX.
I still harbor thoughts of building a small boat out of this stuff, but only if I can get the 12 foot lengths. There is a guy on YouTube who built a small motorized boat/kayak out of CoroPlast. ... https://www.youtube.com/watch?v=K0P-zJDn8ho
richg99