Given your parameters, I'd recommend what I carry in my wallet - the 2"x3", 0.7 oz AMK Rescue Flash plastic signal mirror that Doug Ritter designed. It is lightweight, thin, with a retroreflective mesh aiming aid. If you want lots of practice, and are worried about scratches, get one for practice and one for emergencies - they are under $10. See the links in my post above for my YouTube video of it's flashes from 22-mile range and Doug's writeup on the development. [ The signal mirror in my pack is a 3"x5" glass Viet Nam era military model, though. ]

-rafowell

If it's good enough for Mr. Ritter, it's good enough for me. I'll keep my eye out for one of these, or maybe three. At under an ounce there's no excuse for not carrying one.