I think I saw this idea in a Family Fun magazine or even just their advertisement. Either way we made these up last year, and have had so many positive comments about them.
They are super easy! We just decorated washed out, empty, gallon-sized milk cartons with whatever kind of jack-o-lantern faces that we wanted. Then I put in orange indoor/outdoor string lights. I think the instructions said to use white lights so they looked more like ghosts, but we like the orange, so they end up looking more like jack-o-lanterns.
During the day when they are not plugged in, they do sort of look more like ghosts, so we have the best of both worlds, I suppose!