Hmmm. I honestly don't know of any that make one that's "water-proof."
In the south, yeah, we get rain, but the temps and a seat that cannot breath doesn't sound like a fun combination.
My Corbin is leather, and I treat the seams with a water-proof type product from Kiwi.
Never had an issue, but she doesn't normally sit outside all day/night either so no real place for an issue to develop.
If you have a seat you like, it might easily be less expensive to get a really nice cover or even a small enclosure.
Just thinking out loud.
Cheers,
Steve