Losing the will to live... hope you great minds can help.
My 2.5 yo is just under 15kg at the moment but she's quite chunky and wide in hips. We currently have a britax that reclines but it's only up to 18kg I think and she's already really tight in it - it's one of those deep bucket designs that hold you tight around hips. She doesn't particularly like it but if we're travelling for a couple of hours and I know she'll fall asleep I still put her in it. For every day use/nursery runs I have a basic straight backed carseat with harness. When she falls asleep in that, her head does drop to her chin which looks really uncomfortable.
Now the sticky point. We'll be driving to south of France next month, overnight.
I need a high back booster seat that she can sleep in, one that will not cause her head to drop forward. Maybe one that reclines (dont know if they exist!). Wide enough so she's comfortable and not too expensive as it's a one off trip so I can't justify spending a lot of money on it. With two cars we already have 3 car seats between us! 
Is this all impossible?? Do you know a car seat that would do the trick?? Many thanks!