I used a basic front-facing one and tucked it down between the car seat and the car's seat...the car seat install was tight enough to hold it there for the most part, although I often had to retuck. It worked well enough, although it certainly wasn't perfect.
I thought about using a towel, but I couldn't figure out how to attach it to the headrest. If you're crafty (I'm not), take a dish towel and sew a ribbon in an upside-down u-shape at the top to create a "bib" that hangs over the headrest. This is assuming your back seat has headrests.
Mom to a spirited, red-headed, former 28-weeker 10/2009 and a more mellow monkey 12/2013.