How about this?

http://home.maine.rr.com/rlma/Ground.jpg

I hauled out 180 feet of chain and removed my bilge storm chain locker for a
better look and measurements. This is close to scale.

The ground is shown in red. This would be about two square feet of 1/16"
copper glued to the hull and screwed in way of the ballast fill. A 1/2"
diameter silicon bronze bolt would go through the copper and hull. This is
the same cross section area as the stainless steel mast stanchion.

The connection between the mast stanchion and the through bolt would be a
1/8" copper bracket with bent flanges for resistance to the mechanical
forces of the charge. This would be machine screwed to the pipe stanchion
from the back.

A refinement would be to make the through bolt longer and fasten it to the
side of the bracket with through bolts for a more straight line electrical
path.

--
Roger Long