Description from Martha Stewart
A plank of beautifully knotted Douglas fir creates a naturally elegant tub tray. Cut a board — either reclaimed or store-bought — so that it's a few inches wider than your tub. Sand the wood and seal it with a watertight finish. If you are worried about the board sliding sideways, screw a scrap of wood to the underside at each end, just outside the tub's rim.