Car Clips for Vehicles without Luggage Rack 4pc Set!

The J part of the hook is about as big as the end of your finger. Most (not all) vehicles have a ledge. Some times it's a rain gutter or maybe under the weather molding.

When you tighten the straps over the car top carrier bag, the tension pulls the carrier against the roof and pulls the clips up and holds them in place. The carrier bag must be packed full and tight so gear doesn't shift and loosen the straps.
Pack-Right also offers a roof protection pad which is a good idea for this application. The pad also offers traction on the roof to help hold your gear in place.