When I bought my e84 X1 recently, I found all of our backless booster seats were too wide to fit in the back seat safely. The only substantially narrower one I could find was the Clek Ozzi, and its more expensive brother, the olli. The Ozzi runs about $60 on Amazon, which is a lot for a booster. I took a chance, however, since there was little else out there, and I have to say it's worked out great. I've posted pics of the fit if you are interested. I like that it uses the latch system to stay in place. In retrospect, I think I would have spent more for the Olli, as it uses a quick release system, making the seat easier to remove. On the Ozzi, you have to reach in back and hit the red triggers on each latch, which is a bit tricky, but doable. Also, even though the narrow width lets the seat fit flush to the backrest, it covers the buckle receptor just a bit. You can pull it out to make it accessible, but it will slip back under the seat sometimes. Overall, its pricy, but it fits and my 7 yr old likes it and says it is comfortable. So, there you go.