The first roll of filament I purchased after the small amount you get with your Printrbot was enormous! I looked around Thingiverse for a solution and found this:
Printed out the base, quick trip to Vans and Home Depot for hardware and voila, it works like a charm. I did a 13 hour print on Saturday and was able to walk away without babysitting the filament. The skateboard bearings are a genius idea.