Ok, just got off the phone with my machinist.
Yesterday I sent him a number of different TRS jacks and their inside diameters varied from about 0.251 to 0.257. And one of them had a rather sharp edge at the input. He pointed out that using the O rings perhaps wouldn't be the best way to go given how much slop there is among jacks. So he came up with what I think is an even better idea. A split pin design.
If no one has any objections to this by the end of the day, this is what I'll pull the trigger on. Cost will be the same if not slightly less.