Shower rings might work, it depends on what they're made of and how thick they are. I actually tried this with, believe it or not, ordinary metal binder rings. You can buy loose leaf binder rings at most office supply stores for cheap. They actually did the job ok, but had several downsides:
1. the clasping method was cumbersome and easy to pinch the skin
2. they were too thin so you had to put 15-20 on to get the desired stretch. This created a
weight problem which sometimes pushed the last rings right off the head. How embarassing would it be to have several metal rings drop out of your pants and roll in all directions!
3. took to long to put on, adjust and take off. (must take off at night because they would be too painful for nightime erections)
4. I could only find certain sizes - 2in, 1.5in, 1.25in and smaller. To do good job of fitting a tapered penis you need more options.
If you can deal with that give them a try. I'm looking for something easier.
G