How to prevent my toddlers head from falling forward in carseat?

My 17 month old daughter usually always falls asleep in her forward facing car seat. However, every time she falls asleep, her head always falls forward and she looks extremely uncomfortable. Since our drives are usually short distance, I just adjust her head myself. In a few weeks we will be taking her along on her first vacation, in which we will be driving for a few hours straight. We plan on doing the bulk of the driving late at night while she is sleeping. But I'm wondering what I can do to prevent her head from falling forward for such a long time in the car. I have her car seat reclined as much as it will go, and the seats in my car do not recline either. I have seen those little u-shaped pillows and other neck support contraptions, but I'm wondering if they really work, Any ideas?