Materials for one mirror:
Bottle cap (pick a good one!)
Acrylic mirror (TAP plastics has scraps for real cheap)
Bicycle spoke (free from local bike shop, ask when they cut up a wheel)
Spoke nipple (should come with the spoke)
Glue (I like "E 6000" from TAP plastics)
1 1/8" Hole saw (sans arbor)
Access to a Drill press
Dremel tool (not needed, but nice)
Step 1: Cutting the mirror
Take the arbor out of the hole saw. (That's the drill bit in the middle of it, that helps center your hole. Since that would put a hole in the middle of the mirror, we don't want it there.)
Mount the hole saw and mirror on your drill press. Have a punch-out tool (I use a blunt stick) ready. If your mirror piece has the protective film on it, leave it on! It will help keep the mirror surface good throughout the process.
Cut the mirror out, and then swiftly punch it out of the hole saw before the acrylic gets too solidified. If you're doing lots, do one at a time! Filling up the hole saw with stuck-together mirror bits is bad.
Step 2: Drilling out the bottle caps
Clamp the cap down securely.
Use a sharp nail or the like to punch a small hole to start the hole.
Drill out the hole with a drill bit.
Match the size to your spoke diameter! It's nice if the spoke's threads engage with the hole, it makes it more secure with the nipple. This could be over-engineering, but whatever.