I would like to add the idea of using electrical tape as a laid as friction tape to the inside of the rings. I have used his for a long time and the scope stays in place with a side benefit being no ring marks on the scope.
I don't recommend using electrical tape in there. It (or any other plastic + adhesive film) actually lubricates the joint under pressure, the opposite of increasing friction. You may not have had issues, but that's because there was still plenty of clamping friction for your use, not because electrical tape was a good idea.
I say that from the perspective of a fastener test engineer for a large company. One of the worst things I see in bolted joints is when someone forgets to remove tape or plastic film before bolting parts together.
If you feel the need to increase friction in the joint, use rosin or even Loctite, but not electrical tape.
