I printed this in TPU.  I created this pipe bung because I was repairing my gas fired hot water tank.  There is approximately 5' of gas line between the shut off valve and my hot water tank.  Last time I had it disconnected, it took a long time to bleed all that air out the pilot before the pilot would light.  So I printed this to shove into the end of the pipe to try and keep that 5' column of natural gas in that length of pipe.

I shut off the gas valve and when I disconnected the gas line from the water tank, I immediately shoved this bung into the end of the pipe.  I also wrapped it with electrical tape to help the seal.  When I pulled it off to re-attach to the hot water tank, I spent much less time bleeding air and the lighting went much faster.  So I believe that it worked!

NEVER USE THIS INSTEAD OF A SHUT OFF VALVE!  This is not meant to take any pressure at all.  You must have a shut off valve to shut off the gas.  Just use this to help contain the little bit of gas between the valve and the appliance connection that is at zero psi.

I'd guess it might also keep a copper water pipe from leaking if you had to disconnect something and don't want to drain the pipe?  I haven't tried that.