Tear a long sheet of heavy duty foil, then spray one side liberally with non-stick spray. Slice potato into 1/4" slices, then layer in two rows down the center of the foil, leaving a few inches clear at both ends. Bring long ends of foil together at the top then curl tightly until almost touching the potatoes. Curl the ends in tightly. Grill for 20 minutes over medium-high heat, flipping and rotating the packet 180 degrees halfway through. Open very carefully, as there will be steam.