It depends what kind of car seat you are using... If you are using something like a Britax Click Tight, you just lay a thick woven blanket underneath the car seat, that's what I did... The clicktight locks onto the seatbelt so there is zero movement of the seat... I then used a towel near the head rest, to prevent the seat from denting the seat back...
If you are talking about a carseat carrier, those things tend to move side to side, so you'll want a seat protector that has the grippy surface on the bottom to prevent movement. Otherwise, the thing will slide all over the place when you take curves. For my convertible seat, I bought a mat that was fairly rigid/padded, as I found the thinner ones don't really do much to prevent denting the leather. Of all the protectors tho, none of them extended high enough to prevent denting near the headrest, which is why I use a towel on that portion.