K.T.
When we were faced with a similar situation a couple of years ago we opted for the Cosco Summit carseat/booster. I'm not even certain that they still make it, but at the time it got very high ratings from Consumer Reports and was affordable ... only about $80. It has a 5-point harness for up to 40 lbs. and then uses the vehicle seat belt for up to 100 lbs.
Good luck!