I went to the lake a couple of weeks ago, and I got to try out my new home-made oarlock retainers:
I suppose this post would be a little more informative if I had had some problems, but in fact they worked just as I had anticipated. They dropped easily through the oarlock socket, and then the crossbar settled into its level position. From the rowing position they were completely invisible, and as you can probably see from the photo, they didn’t interfere in the lease with the function of the oarlock.
I then tested what happened when the oarlock was pulled up out of the socket, and the oarlock hung from the retainer. (If I’d though of this five years ago, I wouldn’t have lost that first oarlock at Fort Morgan).