Sorry if I wasn't clear... I leave the cover off of my Roho low profile cushion. I would suggest trying the cushion with cover on and off and see which suits you best. With the cushion cover off I find it easiest to manuver the roho around to get it exactly where I want it while sittting on it in the car after transfer. On the leather car seat it moves pretty easily and I am lifting myself up a bit as I adjust it. It might not shift on you at all when you transfer , with or without cover, and I am only talking about an inch or two in my circumstance.
I did think you are in Southern hemisphere and the middle of summer. A car with closed windows can get pretty hot.
Good luck. I think you are smart to be driving on a cushion.