We use Graco boosters in our Model X. One is backless (and relatively unnecessary for my 10 year old, honestly - but the data says it's safer so we still use it). The other is with a back. That seat has a contact point at the headrest of the Model X as shown here: I had previously made a bolster out of a canvas grocery bag and an unused sofa cushion, but it had a tendency to slide around in that gap, and allow the top of the seats to make contact anyway. Also, my daughter complained about it being uncomfortable, like a lumbar support gone mad. I looked on my shelves in the garage and saw some pipe insulation, and wrapped it with that as follows. Works great! I still think a bolster is a good idea, but this is a solid workaround for anyone looking to keep their 2nd row from being destroyed by a booster.