Hi there guys!
Wendy, I have tried both, with the headrest, and without and also depending on the booster (if allowed) for the best fit. It is just so weird. I have never had a problem with carseats fitting. We have a Radian-RXT in that same car, and no issues, but with the boosters I have tried, it is almost like the car's seat is reclined, even though it isn't. I even tried another Diono, to see if it was just a manufacturer design, but the Monterey didn't fit either, except when I put it in the reclined position (which it says to do in the manual to get a snug fit), but that also makes me feel weird because I have also read that it isn't ideal for safety to have it in the reclined position. I just want a booster that fits, without having to do anything special to it
Wendy and Murphydog, perhaps I was misinformed by a local CPST when I had inquired with my question about this issue. She had told me wedges were available from some manufacturers to remedy the gap. I honestly did not even look into it since I want to find a high back booster that fits my car the best and is safe without having to get something extra to fix the gap. The wedge she mentioned just made me feel uneasy, so I didn't pay it mind and took to here to see if anyone had a better answer for me. It sounds like if there even is a wedge option, it is not well known, which convinces me even more that I want to stay away from that option.