The ready made ones I've used are called Dynaplates.
"A matrix of bronze spheres greatly increases effective area relative to flat plates. Use them as electronic grounds for SSBs, Lorans, and Ham transceivers, or as a ground reference in bonding systems. Includes gold-plated bronze flathead fasteners and silver contact paste for superior conductivity."
West Marine has them and the 18" length model number is 196040. Pricey but effective.