My niece is XL and her hips get pinched in normal seats and she can't fit in a harness. We have had the Radian, Britax Frontier, Graco Highback Booster, Britax Parkway high back booster, etc. Her shoulders and hips don't fit or she is over the weight allowance. She is about 55inches tall, 114lbs of solid mass. (Before anyone judges, she is under medical care has a strict diet, tons of activity etc, it is something we are on top of but it is how she is made). What can we do to keep her safe in the car?