I used a speedrail straight fitting with the correct inside diameter. Then then the out side diameter is just right so you can mount NRS oar towers by grinding a groove for the u-bolt where the hex bolts are inserted. You use the hex nuts to hold the fitting where you want it.
no image I know of, just a short piece of tube or pipe 4 to 6 inches long with an inside diameter that fits over your frame and the outside diameter the size you need for your oar lock. I have made these in the past. I cut a slit length wise in the shim so that the u_bolt clamps the shim tightly on the frame.
Curtis I had to. The outer pipe was too loose on the inside pipe so I ground down a PVC pipe and created a sleeve to fill the gap. Well I didn't account for the slickness of the pvc. At full tightness it allowed some movement so I said to hell with it and drilled it. Rock solid now! ?