Top of backrest?
Tie a taut line from the rear deck fitting forward to the carrier.
A warning about tie downs and straps in general. Many bow and stern tie downs come with open hooks at the ends. We’ve had three of our members have kayaks destroyed, and in one case broke their windshield as well when the hook came loose and got caught by a tire. Two of the boats were Kevlar, one only a few weeks old. The ends of long kayaks can bounce quite a bit on a rough road as the racks and saddles flex and the hooks come loose. Beware of loose strap ends as well.
I’ve used a vice to close these open hooks and added carabiners or snap hooks.
That is exactly why my husband and I only used the tie downs that came with the Thule stuff once. A couple of blocks from home one came loose. We pulled over but realized it could have been really bad if it had been caught in a wheel well.
In general that is why I prefer hood loops up front…
I love my Thule QuickDraw tie-downs but was surprised to see they came with hooks. I replaced those with quality mountaineering carabiners.
This is definitely the thing that nightmares are made of. @Celia When I got tie-around straps (if that’s even what they’re called lol) at REI, the guy recommended using the under the hood loops for the bow line. While I wish that would work, I’m not sure if it would though due to the way the Mini hood is. It’s not flat across, the edges kind of come down to the fender. I’d be afraid that pressure of the ties would bend it. I may use them for the stern though and loop them through the trunk.
The bump is the top of the backrest
If hood loops are not a plan w your hood, consider doing what Rookie did. Bow line IMO most important of the two. You have to be able to see early and often if something is coming loose up there. The bow line will usually show that to you.
I have seen people in vehicles rear hatches or liftgates run the stern line to clips or something on the bottom of those. I can’t vouch for what they use but it is clearly possible. And it again shortens the length if that line reduce the possibly if it bouncing into a rear wheel well. I have seen lines long enough to possibly do that bouncing along the highway, unfortunately never in a car l could hail.