Oar tethers

I've been using home made oar tethers for quite some time. They let me down or more to the point, hindered me, last trip. So I have been looking around. Cascade Outfitters sells these:
https://www.cascadeoutfitters.com/images/products/0347.jpg

Anyone using them? Like them? What is the inside diameter of the ring that rides the oar shaft?

Thanks,
DanCan