Searching the forums I found lots of threads where people had this same problem, but never found a good solution. Well, I tried my daughters Graco 4ever seat that turns into a high back booster, and it actually seems to fit very well. The headrest is offset several inches from the back of the seat, so it fits the contour of the Model S seat very well. Only issue I see is the headrest is almost touching the roof, and my son is only about 45” tall...but this seat should work until he’s big enough for a backless booster.
Just thought I would pass this along! Anyone else find a high back booster that works well?