Two straps, foam blocks and if it is a bare naked roof just run the staps around the boat and thru the doors to clamp inside the car. I am assuming you have tripped over cam straps, if not go find them.
Use the handles on the ends to restrict the boat from going side to side via bow line to under the car's bumper, or you can put loops onto the hod bumpers and create a V-shape with a long rope. Stern rope if you can find something to tie to. but with a good bow line you'll see soon enough if it is starting to move.
I completely agree with the concept of not relying entirely on the handles - for any boat no matter how well installed. You may need those to aid in an on-water situation, as in a capsize, so it'd be silly to wear them out just getting the boat to the water.
URCHIN Portable Anchor
Full Size Sail Rig
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