Bhautik Joshi Guides us through a simple but very fun DIY project:   "Built using a fisheye peephole as the main lens element and a decapitated soda can as the lens body (!), this attaches directly to my SLR camera. For well under US$20, I ended up with a lens that has nearly a 180-degree field-of-view, adjustable focus, a canon EOS mount, and due to it's stylish and sleek exterior, can generate limitless amounts of admiration ridicule confusion. Unlike commonly (and cheaply) available fisheye adapters, this build does not attach to the front of an existing lens - it's a completely self-contained unit that doesn't require any other optics to work."   Link