Back in the 1980s, there was a gun magazine called FIREPOWER which showed how to make full auto firearms and silencers. For a silencer, it was found that two liter soda bottle hose claimed on the end of a .22 did a good job for several rounds.
Well lookee heah, lookee heah! I still got those old FIREPOWER magazines!
Mythbusters showed that a pillow was a very effective suppressor.