Ok, all the praise this idea has received convinced me to try it. Went to Home Depot tonight. They had more O-ring sizes than one could dream about, lol. I picked up the mentioned #60 and #61 just to see and feel the difference. They were also sold individually. Each only cost me $0.17.
Since the two were only one size different, not much could be seen with them next to each other. However #60 is slighly thicker than #61. End result is very comfortable cushion when put onto the AR/GB thumbstud. #61 will also do the trick, but the ring around the thumbstud will be a little thinner and obviously just a little looser since it's one size larger.
Between the two I prefer the #60, but take what you can get. Both will get the job done. With one of these O-rings on, it's just as comfortable as a disc opener.