Any recommendations for an inexpensive harness to high back booster seat?

We have Cosco Scenera for my parent's car, but the max weight is 40lbs. E is currently 38, so we're quickly approaching the limit. They rarely drive him, maybe once every other month, so I'd like to get a new seat that's under $100. All I can find in that price range, however, has a 40lb. limit on the harness. Umm, he just turned 3 and is actually still rear facing in our cars, so no way he's going in a booster seat.