I used the same circuit, but made some design adjustments to the casing and the speaker choice. At first I used a speaker that I ripped out of an old telephone, but it overloaded in about 30 seconds. So instead, I used a 2.5 inch, 8 ohm speaker from You-Do-It Electronics, which ran me about $2. I also found a pretty good looking box for my circuit lying around the house. Here is the first edition of the assembly:
I used a cup to amplify the signal a little bit more. However, after poorly soldering the circuit together, I had to re-do the whole thing and I ended up with a better result. Here is an image of the new version's guts:
This little thing can pump a jam nicely and should work ever so well on the "yet to be named" robot.