I was asked to do this as a magazine article but I just don't have the time at the moment so I'll do it this way so my Blog readers get the first crack at it.
This little rig is easy to build and works quite well. A commercial one will cost you around $50 while the only real expense I had was the bottle cutter which is just a couple bucks if you have to buy one. The other parts were just made from scraps around the shop. I didn't give any measurements as the bottle you use might be different than mine just as your taste in the beer inside might be different than mine. I don't think you'll have any problem figuring out the sizes.
There are lots of photos and explanations in the Gallery to help you build it. Good luck. Comments welcome.