I found a pretty good way to protect the mirror in my wilderness PSK.
I scratched the screen up on my old PDA pretty badly. When I got my new PDA, I discovered that they make little screen protectors that just stick to the screen. When I went to get the screen protector, I discovered that they would not sell me just one. I had to buy a package of 12 for $20. Well the one I put on my PDA works very well so I tried it on my mirror and it sticks there very well too and prevents scratches.
I don't know that it would stay on if you were carrying the mirror on a lanyard or something. My mirror resides in a tin and this protects it there. These things are about 3 inches by 3.5 inches s they fit my little 2 by 3 mirror with room to spare. Might try multiple protectors on a bigger mirror.
I need to start one on fire and see if it will work as tinder (I suspect it will).
Any way, before you go drop $20 on a dozen of them, I have 10 left. If you want to send me $2 and a stamped, self-addressed return envelop, I'll send you one. PM me for my address. I'll even stop by and grab more packages it there is all kinds of interest.