Slice a small seedless watermelon 1/4 inch thick; trim off the rind. Cut the slices into 1-by-3-inch rectangles. Arrange on 2 wire racks set on baking sheets. Bake at 250 degrees F until almost dried but not browned, 3 hours. Reduce the oven temperature to 200 degrees F and continue baking, rotating the pans and flipping the pieces every 2 hours, until the fruit is leathery, 3 to 4 more hours.