We have a 2017 Model X 6-seater and a 62 lb dog. We just finished a road trip with one extra-long day of driving up and back. On the trip out I had a seat cover for each of the second row captains chairs and expected the dog to stay mostly on the floor.. The poor dog remained sitting up in a seat panting for the entire drive.
While at the destination I purchased a "seat extender":
The main feature of a seat extender is a hard bottom that creates a floor from the back of the rear seat all the way to the back of the front seat. For the 6-seater it also creates a floor between the two rear seats. This is a generic-fit model, but it fit fine in the X and had one-day shipping. I also liked the large mesh area. It has door protection as well, but I just let them fall over the outside of the seats. There are other versions that looked like they would work. The stiff floor uses three boards (sort of a hard cardboard). You can fold the cover up to expose one of the second row seats and access the third row if you need to. We had four people and the dog for a couple of short trips. Install is easy enough, but a bit more work than a soft cover. It does fold nicely for storage.
I added an inflatable pillow between the rear seats for a little extra support:
This looked like a useful thing to carry around in the car, though this was its first real use. I don't think this was necessary for support, but it didn't hurt.
Our dog was able to lie down for most of the trip back home, looking at us through the mesh most of the time. She was very relaxed, with no continuous panting. We were all very happy with this thing. The dog felt the floor was stable enough to walk anywhere on it without worry. An added benefit was that the dog could brace against the front seats during sudden stops instead of falling off a seat.
If you can put up with the size and restricted floor access (which actually wasn't too bad), this is a much better solution than a soft seat cover.
ETA: The center console A/C vent ends up below the extender floor, the one thing I'd like to fix. The side pillar A/C vents do blow air into the dog's area. Our dog loves to sit in front of the A/C, but didn't seem to miss the center vent.