Friction between car seat and child seat does nothing for a side impact. You need to tighten the hell out of the seat belt against its ratchet (if that is what your are using), or the LATCH harness.
Installed properly, you will put divots into the leather or fabric, and cushioning. A cover or towel does nothing to prevent this in a tightly and properly installed safety seat. All a cover or towel does is temporarily keep barf, spilled food or drink from first touching the automobile seat. If you can tug the child seat on any seating surface (fabric, leather or a towel on top of either), and that child seat moves in the least, it ain't tight enough, period.
If you physically cannot get on top of the seat (get into that crouch), jam one knee into it, and put your entire weight into it to get those belts (or LATCH harness) as tight as humanly possible, then anything less than that is an unknown half-measure. In that case, get somebody who can; no shame in that. The policeman at a Seat Check Saturday can do this...yes, he will jam the child seat into the automobile seat deeper than you thought possible, and it will look like the automobile seat surface will most certainly be damaged....I say screw it, go all the way, you can replace the seat cover. What you want is the belts securing the child seat as tight as banjo strings; same if you are using LATCH.
Too many people worry that "a towel might cause it to slip", and then when the child seat starts to really crunch into the automobile seat as they tighten things down, they back down for fear of ruining the automobile seat. Wrong answer. The hell with damaging the automobile seat surface or not; you can replace it when your kid outgrows that child seat. You can't replace your kid.
OF