I have '20 crosstrek and just got myself this air mattress on Amazon.

It arrived today and I tested it out in my driveway. It fits pretty well for a generic car air mattress. It has different chambers that can be inflated/stay deflated depending on the size of the vehicle. For my '20 crosstrek I inflated all of them. The side chambers (the 'tubes' that are kind of sticking up in the picture that run the length of the walls) I can semi-inflate so that they don't stick up as much and when I lay down on them they squish into the gap to form fit.

The 'head chamber' (the section closest to the front of the car) can actually fold down while inflated to run the length of the rear-foot-well from left to right. It isn't a perfect amount of support but it definitely is enough to sleep soundly for me. And if I bolstered it up with just a backpack or bag it would be enough.

What's nice too is that I can imagine on hotter nights to not close the rear-hatch, to pull the air mattress down over the rear-bumper, and so I won't even need to worry about the void of the foot well since my head won't be that far up.

I've also found that sleeping with my head to the rear of the vehicle (even with the hatch closed) is just enough room to sleep with the void. My feet hang off of it but it's fine. (I'm 5'9" by the way).

Now if you want to splurge you can get the Luno Airmattress which are made for specific vehicles. They even include inflatable pieces to stuff into the seat wells as seen in this picture. They're way more expensive than what I purchased and honestly I'm glad I didn't because 1/4th the price is amazing considering how little room for improvement the air mattress I got needs.

Any questions just let me know.