I'm not big on making things myself aside from computers and messes. When it comes to large DIY projects like car work or building things, I farm them out to people I can yell at if things go wrong. If you are the DIY type who likes to take digital photos this may be a good project for you.
A research engineer from Industrial Light and Magic has conjured up a very ugly fisheye lens for his DSLR, but the thing works well and was cheap. The guy used a single element film projector lens, peephole, foam, hose clamp, and a lens adapter from eBay to cobble the lens together.
As you can see it is ugly, but the images it takes are impressive. The whole works cost under $20 apparently. If I made it the thing would be even uglier and be coated in blood where I cut myself on the can.