how tall is your piece? what method of cleaning you are using? they sell pieces to plug up the downstem for this purpose. but im not too positive on the mouthpiece.
depending on the tube size you get buy a pack of water balloons and stretch that over the mouthpiece and it would create a pretty much air tight seal
Just 20 inches tall. 2 inch diameter mouthpiece. I want to plug the mouthpiece so I can shake my bong upside down to clean because of percs.