Im glad to hear someone mention Coroplast, which is the trade name for corrogated plastic. Think cardboard box made of plastic.
I have been using this stuff for years, for everything. I first learned of it when making RC airplanes from it, of all things.
It really is an unsung product. My current brooder incorporates more than little of it.
I get it in all sorts of ways. I have purchased it in 4 x 8 sheets at a sign shop, which is cheaper than ordering it online. It comes in white and many other colors.
I pick up roadside signs, the little "bandit signs" that sprout up everywhere.
The large candidate endorsements/adverts seen at election time are 4 x 8 - and, win or lose, are useless when the election is over!
** BTW, these are private property, belonging to the campaign organizers, so be mindful of that. They are normally relieved to not have to go take them down.
Stores use a lot of it in advertising floor displays and then throw them out - so I recycle them.