Clek booster seats
The Clek booster seat latches in like a infant car seat and fits perfectly on the inside of the actual seat. It is a few inches narrower but still has arm rests that my 8 year old daughter sits comfortably in. I am by the book when it comes to these things and while many at her age don't use the bosster, I will until all the factors are met. Protective dad!
Anyways, I have one of those car savers that I slide underneath...it is actually the torso portion of the one I bought for my 3 year old's car seat in the R Class. It fits perfectly underneath and the great thing is once I'm done it goes in the trunk. One warning that I have is your kid probably won't be able to click it in as it is a tight fit but you will be able to do it.



