you need a "H" clip i call it and it turns your retractable seat belt into a lapsash style seat belt so it cant move it is fixed at one length. this si then put through the base of the car seat and if you sit / kneel on the sit fit the seat belt so it is sooo tight you can only just get the clip in, you will have minimal movement.
this si how the lady who installed DD's car seat showed me as we were moving the car seat from car to car regularly for a few months.