Harry....
What I have done on my 22 is to buy a five dollar set of copper jumper cables and, when conditions get a bit on the thunderstorm side of things, clip one to each chainplate under the spreaders and let them dangle overboard.Of course, the most important thing I do down here on the summer thunderstorm prone Gulf Coast is to (1) avoid those conditions of possible and (2) crank up the motor and run like hell when needed.You could, if really inclined, drill a hole in your hull and intall a grounding plate near the mast step, but I'm not sure it is going to be any more effective.