Harness boosters that are easy for carpool

Hi Ladies - we are carpooling for preschool this year and I was wondering if anyone has recommendations for a booster that is easy to get in and out of the car.  My son is not over 40 lbs so I will need to go with a harness one.  I was just wondering if anyone currently does this where there is a seat that gets moved in and out of cars a lot.  Thanks!