A few months ago, I came across an Instructable on making a compressed gas cylinder wind chime: http://www.instructables.com/id/Compressed-Gas-Cylinder-Wind-Chime/. Using it as a starting point, I started working on my own.
It turned out pretty cool.
Step 1: First Steps
I researched the length to diameter ratio of temple bells and found that most of them were approximately 2:1 length to diameter.
I cut the bottom of the bottle a little at a time until I got close to a 2:1 ratio.
Step 2: Finishing the outside of the bell
I also removed the paint at the top of the bottle, and ground off the numbers that had been stamped into the metal.
Using a cut-off wheel on the die grinder, I cut off the valve mount. I then returned with more Scotch Brite and smoothed it over.
For a final touch, I used a swirling pattern with the Scotch Brite to give it the pattern on the outside. People have told me that from a distance it looks dented at first, but it's actually very smooth to the touch. It has a nice, almost iridescent, look in sunlight.