You may laugh, but one of the best featherweight mirrors I've found is this polycarbonate job from Coghlans:

http://www.rei.com/product/745496

I bought about ten of these on sale, and they hold up remarkably well. Each kit has one. Good reflective image, little distortion, and they last. Though they will scratch without a bit of protection.

I even use one as a stiffener for my passport while travelling (leg wallet).