im not sure witch signal mirror you mean, but if you got a "feather weight" mirror it will do just fine. it's a good compromise of size/weight and effectiveness. atleast thats mine mine opinion.
if you got a big and thick mirror throw it in your pockets.
most guys don't put a fullsized signal mirror in the tin, they take to much space. usually the fullsized mirror is carried in something else (pockets). A improvised mirror is usually throw in a PSK to conserve space and weight. usually just some shiny sheats with peel-off sheet to protect it and a small hole in the middel for aiming. if you can't find it you take alumium tape and polisch it with silver polish, put the tape on peel off backing or tape it on your tin.

just mine 2 cents