A flat pattern that can be scrunched into shallow conical rings which approximate a parabolic reflector (focuses far-away light sources into a small area). Focal length and diameter are both about 11".
UPDATE 2018-06-18: I have added an Octave script that generates flat patterns for other geometries.
Cut this pattern out of something shiny and flexible, like thin mirrored acrylic or metalized plastic film. Then tape the two ends of each arc together, which will pull the flat arc into a shallow cone.
If your material is too flexible you can cut out some backing pieces out of a cereal box or something similar using the same flat pattern. If it is not flexible enough, you'll have a hard time pulling the pieces into cones.
Tape or hot-glue the rings to a backing board to keep them all lined up and concentric. Be careful not to distort them when you attach them to the board. My example pattern gives me about a 3/4" wide focus blob. This works best when your target object can be put on a stick, so you can hold it out over the reflector without blocking the light with your hand and arm.