I was thinking along the same lines. May be use a 2x8 and nail a small quarter round molding along the edge to prevent getting off track (or on both sides). Also, you can buy a roll or strips of non-slip tape to run down the middle to give some traction especially if the tires are wet. For storage, maybe just flip the 2x8 on its side so it only takes up 2 inches on the side of the stairway.