I too was shocked at the $70 price tag for the external counterweights, so I made my own.
I used lead strips sold as 'duck decoy weights', like these...
To test them out I just duck taped them to the oars. Later for better looking setup I encased them in fiberglass and added some nylon set screws so they can be easily attached and removed.
I used 8" woven fiberglass seam cloth and some systems west epoxy leftover from a kayak repair. I found some PVC pipe with the same O.D. as my oar shafts to use as a mandrel. I wrapped the PVC in plastic stretch wrap as a release agent, then a layer of wet fiberglass cloth, next a layer of lead strips, another layer of wet fiberglass cloth and finally a layer of stretch wrap to hold everything tight while the epoxy set up. After it was all cured I drilled and tapped holes for some nylon set screws.