Amazon has this high-rated 100% waterproof seat cover for pets for just $15.99 when you use coupon code EKH7QCIQ at checkout (regularly $25.49).
This rugged seat cover has 4 layers of protection. It measures approximately 54×58 inches, so it’s the perfect size for most cars and family SUVs. A combination of headrest strap, seat anchor, and the non-slip layer enclose a stable area for your pet to stand or sit on when the vehicle is moving. The Velcro opening also lets seat belts come through, so the humans can still sit in back, too.