Hello! I have 2 Diono Radians - one in each car. We flew to Disney last year and didn't need to deal with rental cars or car seats. This year we are going on vacation and renting a car. By the time we go on this trip he will be 5 and he will probably be 50 lbs. I can't see lugging the very heavy Radian through the airport. I want him to be safe and would purchase a lightweight 5 point harness to take with us on the plane for the rental car if i could find one. Any suggestions? is he really big enough for just a booster?