Heavy duty 'robust', heavier than the typical imports sold in hardware stores.
Brass ground rod clamps, aka Acorn clamps.
U.S. manufactured by EMC (Electric Motion Company, Inc. in CT).
For 5/8" and 1/2" OD ground rods. The photo shows some of the clamps with a 5/8" OD mast. $3./ea.
Plus $6. Priority Mail.