M.G.
We're taking our kids on an 1100+mile trip in two days. Our 5 year old is in a high back booster also without the comfort to lay her head down. What we're planning on using is her pillows with blankets placed underneath for bulk. What we have wished we would've picked up are the actual child travel pillows which you can find online and at many stores such as Babies r Us. We've also found that our daughter can make herself quite comfortable on her own so long as she is kept happy and for her that means having her ipod to listen to. Not sure if your little one can keep herself comfy too but it amazes me just how comfortable kids can make themselves in the car when us adults are anything but. Best of luck.