Not exactly sure this counts as MYOG, but since it took some screwdrivers I'll count it as such!
Dont want to argue whether one should carry a mirror. I choose to do so because I like having one for signaling, tick checks, and the occasional personal grooming. I have carried an Adventure Medical signal mirror for a while but I find they scratch easy and do not have a great reflection.
I found a much better option - a platter from a bad hard drive. I pulled two out of a laptop drive recently. They are very reflective and VERY lightweight. This one weighs in at only 4 grams. If you have any bad hard drives laying around, open it up and give it a try.