A chunk of rubber.
Timothy, I just looked at 1" X 12" X 12" hard rubber on the McMaster-Carr site and it's out of this world but something that might work, isn't. Still into a local truck tire facility and ask if you could cut several thick cleats from one of their junk tires. I'll bet you could make your own 3" X4" plug from the material, deep enough to do the job. Also, you could secure the plug in the tube box, then drill a couple of holes or four for that matter, near the top and use stainless steel sheet metal screws driven right into the rubber to hold it secure. 3M Scotch-Weld Neoprene Rubber and Gasket Adhesive or 3M Company 8001 Yellow Super Weatherstrip Adhesive. I can attest from personal experience that the 8001 adhesive is beyond aggressive, once it's on and cured, it is near impossible to break the bond without starting to destroy the rubber. Maybe vou could stack the rubber chunks to make a good and deep plug. Actually this is starting to should like fun and a challenge at the same time. Can anyone else suggest a source for Timothy to get his hands on some thick rubber? Glyn