Below you will find some shots of me cupping an old fruit jar.
I prefer a 2 1/2 inch Mason jar. If you can't find a Mason jar,
 a water glass will work just as well.
This is not only great for muting the harmonica,
but it is also great practice if you intend on using a bullit mic.
Since most bullit mics are around the same diameter as a fruit jar.
By opeing and closing your hands , you can make a great waa waa effect.
cilck on the sound bite if you want hear what I sound like with Fruit jar
Give this a try and have fun.