I own a ready made mirror, but it is too bulky to fit into my survival kit. So I made one out of hard disk plate, cutting a part of it and drilling a hole in the middle. The rationale behind is as follows:
+ lightweight - it is an aluminum alloy about 1.5 mm thick
+ effective - the surface of the plate is almost perfect. It means almost all the light is reflected. The surface is perfectly flat, so that the resulting beam is very narrow and thus visible from longer distance.
+ drilled hole allows sufficient sighting. It's double sided, so one can use the "dot on your face" method when it is possible.
+ it should be stored with some care, although it is surprisingly strong. I protect the surface with an peel-off tape.
It should be noted that I used a plate from an quite old HDD that was silver like. More recent drives I disassembled had the plates a bit brownish, what makes them less effective.
I'm pretty happy with this solution, it is small and lightweight but fully functional.