I have seen tree skirt patterns in two older McCall's Quilting mags that were just a square quilt with a hole cut in the middle and a slit cut to the hole, and ties attached. One had the slit cut from the corner to the center, and another from the middle of the side to the back (so one was "on point" around the tree, and the other sat square to the tree).
I think I'm going to try that for my tree this year.