My 65lb Golden rides on a custom seat cover in the back seat, but honestly (and sadly for both of us), she hates the Tesla compared to "her" former Lexus SUV. Primary issue is MS just does not have the airflow she desires in the back seat. Range Mode OFF helps with a few more points of fan speed, but in doing so, also blows the people in the front with the same gusto unless you play a lot with the vents. What does make her happy is fan on max warp, side window open and pano open -- but that's not very realistic for anything but short jaunts around town. With my MS, she only comes with me on short jaunts and when it's very cool outside. As summer approaches, I'll be in for a lot more of those big eyes when she has to stay home more often than not.
I have never put my girl in the back trunk area, primiarly because of comments from a number of people related to the rear jumper seats and lack of ventilation in the back for their kids, made me avoid even considering that option. If you do put your dog back there, I'd give consideration to tint on the back hatch, perhaps using the Tesla sun screen, and/or some additional ventilation fans like people have done for the kids in the rear.
So, net of all that, I'm sad to say, is if your dog is sensitive to heat or requires a lot of airflow to stay comfortable, you may be in for a challenge with your companion riding in the back of MS. Maybe you can arrange for an overnight test drive to gives things a little try... Good luck!