Alright, I'm too cheap to spend $5 on a plastic extension cord wrapper to keep my Christmas lights from becoming a tangled mess over 11 months in storage. I had a heaping pile of shipping boxes from all my on-line gift purchases though, and I had to do something with them anyway. Maybe it's an obvious solution, but sometimes those are the ones that get right by us.
Step 1: Measure, Cut, and Fold
Cut a flap or side of a shipping box, from about 6 to 11 inches long and 4 to 6 inches high. Orientation of the cardboard corrugations is important here, we want to keep it vertical so you will see that wavy "S" pattern in the long measurement of your rectangle. This adds strength to the outside "ears" of the wrapper.
Make two inward diagonal cuts on each long side of the cardboard, about an inch in from each end (see diagram). These cuts will allow us to fold the cardboard into thirds and result in a flattened spool shape.