Sprinkle the board with a generous amount of kosher salt. Squeeze lemon juice over the salt, and use the cut side of the lemon to rub the mixture over the board (if your lemon isn’t very juicy ...
The tree might not look as full and even as it would if you had strung lights vertically, but this method can cut down on the headache if you value your time a bit more than the look of your tree.