The majority of car top kayak anglers in my area don't bother to flip the kayak upside down as recommended. They just slide it up on the roof racks and lash it. A buddy of mine used a raised T-bar on a tow hitch for his 300zx sports car to make a more level lash area on the roof.
If you want to get fancy there is stuff like this.
https://youtu.be/IrSCc5wX_TM
Good luck