Thanks, BargainFittings.
I picked up the Delrin ferrules at Home Depot because it was easy, although I'm going to get some o-rings from you when I place an order for some other stuff to see how I like them. I am surprised that they are 1/2" OD, though. The inside of my compression nut measures exactly 0.500", but the opening of the tapered part of the fitting (where the ferrule would normally reside) measures quite a bit less than 0.500"... something around 0.450" if I remember correctly. Have you ever noticed that an o-ring got sliced by the sharp edge on the top of the tapered surface after you took a sight glass apart? It's probably not an issue...