Step 1: Parts List
You will need:
1. A peep hole - the kind you use on your front door to see who is on the other side. Get one at the hardware store. They come with 160 degree or 200 degree field of view. The 160 is probably fine for most wide-angle work. The 200 is as wide as most people will ever need. You can actually see slightly behind the lens with this one. And, it works better for fish-eye effect. However, is may show more barrel distortion. If you can't decide, get both.
2. PVC Pipe. The smallest diameter you can find that will fit around your camera's lens barrel.
3. Craft foam. These EVA foam sheets are available at craft stores and any place that sells scrap booking supplies.