[67SS4spd]

Find a cheap small mirror at the dollar store and its most likely 1/8 or thinner. US is 3/32”, 1/8, 3/16, 1/4 etc.
The 3/32” is referred to as single strength and it’s not to common. You should be ok with 1/8” if they grind the edge and seam it pretty so the seam is at the thickness of the metal frame lip, it will still look good. Make sure the back gets mirror sealer, glue on a with DOW 795.
Steve (glass guy)