Is there any sort of product out there that is just, like, a piece of waterproof canvas that can attach to your belt and hangs over your butt so you can sit down while /out/ when it's damp and not get your ass soggy?

I was thinking of just making one but it'd be easier to buy something if it exists. However, I have no idea what something like that would be called in order to Google for it. This has to be a common enough problem that SOMEONE has found a solution to it.

Pic maybe related?