Third, by total accident, I found out that plastic fill valves for toilets, purchasable at any hardware store, are the same ~1 inch size and thread pitch, as air mattress valves. They even taper to a smaller size that comes close to Schrader size. This means that you dont have to create your own Boston valve, if you just so happen to be repairing toilet valves as I was.
Pick it up and wrap it around the valve, using fingers to help as soon as it's cool to touch. Don't get it into the screw threads or the hole at the tip. Try to make the lump of plastic slightly wider than the inner diameter of the air mattress valve.
Grease the inside of the air mattress valve so that the plastic won't stick. When it has cooled down to about the hardness of plasticine, stuff it into the air mattress valve and hold it there until it's hardened.