We will be flying next month with our 6.5 yo. She's pretty big...90/95 percentile for hgt/wgt. We probably won't check luggage; we will depart late Thurs and return on Sun, so she'll need the seat for 2-1/2 days in a rental car. For sake of convenience, would it be okay to just carry the backless portion of her TurboBooster? Or should we suck it up and carry the HBB?