What you'll need:
1) An SLR or a DSLR camera body with interchangeable lens.
2) An oversized lens. Ebay is great resource, you can pick a high quality lens up for around $15.
3) A rubber accordion-like plunger (preferably a black one, to prevent light leaks, but if you can't find one you can use a different color one like me and just spray paint the inside black).
4) Plastic body cap for your camera.
5) Hot glue gun or other adhesive (super glue optional).
6) Drill, dremel tool, or hobby knife to carve out center of body cap.
7) Black electrical tape.
First cut out the center of the body cap with your dremel tool or hobby knife. If you only have a drill, you can drill lots of holes around the edge of the cap until the center pops out.
Cut the plunger to the correct size. Fit the lens into it and see which ring it fits at, cut there. See where the body cap fits, cut there.
Insert body cap into plunger and glue liberally.
Attach lens snugly to plunger, apply fancy high-tech hot glue to hold in place.
Tightly tape over the glue to hold it in place and give it a nice black finish (so your scraggly hot glue bits aren't sticking out everywhere.)