I live close to some cross country ski trails and don't always have access to a car. At first I used velcro straps to place on the top tube. It worked well enough except when the temperature drops to below -20C it's a bit too cold to put back on the bike afterwards. Someone at the local bike co-op made a PVC pannier attachment so I went to Home Depot and bought the supplies to make my own too! Much faster to pack up, only trouble is fitting through doorways and the elevator at my apartment. Have to take ski's out when going indoors.
