For my friend's car (and I think I'm going to get one for mine too, because it works great in hers), we got
the Sunshine Kids Grip cover thing. It's hard to tell from the picture, but it gets really narrow at the seat bight part (narrower than it looks in the picture). We just cut off that bottom part, so it's only the seat back. It's completely waterproof, and whatever dirt is on shoe bottoms kind of sticks to the grippy material, so it doesn't shower down onto the carseat (if that makes sense).
It's also really thin, but not remotely something that I would use under a car seat during installation. Also, for people with texture issues, it's kind of squicky, because of the rubbery-ness. I don't touch it very much