Yesterday I had a chance to shoot just a few rounds (for the first time) from the copy of the Chambers rifle I built a while back. Got ready to clean it today with my usual remove the lock, plug the touch hole with a toothpick, fill the bore with water, etc., etc. Just after I put the toothpick in the touch hole, I broke it off and spent a while trying to get the remnant out. As I was doing this, I noticed that I had two high pull, small magnets on my work bench and had a thought. These magnets are rare earth (Neodymium) and are 3/8 inch in diameter and 1/2 inch long. They have a pull of 19 pounds. So I cut a small piece of silicone tape to use as a gasket, placed that over the touch hole, and then held it in place with the magnets. Long story short, it worked like a charm. No leaks. Easy to attach and easy to remove. The magnets are commercially available. To each his own but this is the new normal for me. Thought others might like to hear about it.
Lock out, magnet and gasket, plug in place, all done.