You could use Latch or buy a couple of these: https://www.amazon.com/gp/product/B07WMTQHCL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
My daughter will have to move to a booster seat for various reasons when we travel, so I've been thinking about this a lot. I learned they make safety vests for the car. This is one. The only issue is it feels like it would be a PITA to use with a compliant kid and hell if the child is not compliant.
Thanks for this. Forgot about the change of clothes.
BTW we just got this stroller, and this harness as a car seat replacement.