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They work ok. There is a plastic buckle that wraps around the top of the seat. On my Next Gen seats, I noticed a small indention where the buckle was due to it being a little tight even at the longest setting. I simply unbuckle the mats when my kids are in the car and buckle it back up when they are not. Bottom part of the mat is secured by an elastic band that goes over the top of the seat without any issues.
My kids still put their feet on the seatback, but at least they're only getting the kick mat dirty!