We now have available a collapsible ground rod kit.
This kit has 3 36-inch x 3/8-inch ground rods. Two 5/8 couplers and One 3/8-inch bolt.
The rods are threaded on each end and one end is pointed. You can drive a rod in the ground using a coupler and bolt, then take a reading. If your results are too high, remove the bolt add a second rod, coupler and bolt. Drive the rod deepter. Take another reading. Again, if your numbers are too high, add the third rod.
This kit comes in a PVC Pipe case with threaded ends. Overall length of the kit is 40 inches so it will fit in a compartment.