Dave, I just thought of another alternative that I could try very easily. In the past, I have used this type of plug on two 3 gallon plastic carboys that I bought used and which already had holes drilled. This is inserted into the hole and the wing nut is run in, pulling the screw head and washer (on the inside), expanding the diameter of the plug. It seems to work well but I have not used the 3 gallon carboys because I am concerned about the metal screw and the black rubber. They both may be okay, but I have not pursued that.Cool thank you for the follow up… I imagine that since you placed it from the inside with the large end of the taper, more pressure on the inside would tend to ensure a better seal. Interesting idea with the SS finishing nail. I was looking for a threaded plug and will post my findings if/when I hear back from a few suppliers.
However, I do have more #5 stoppers that I could alter and use similarly. I would need to drill a hole through the stopper from top to bottom. I would put a stainless steel screw and washer against the large diameter of the stopper, feed the stopper through the 1" hole from the inside of the carboy and attach a washer and wing nut on the outside against the small diameter of the stopper. Tightening the wing nut would increase both inside and outside diameters, sealing the hole. I think I am going to give this a try because I like the mechanical attachment.
I am sure there are a number of people reading this thread and thinking, "Why doesn't he just deep six the old carboys with holes and get new one?!" Where is the fun in that?