The silicone caulk method is well accepted. It won`t warp the mirror if the thickness is more than a quarter`s thickness for sure.
All you do is put 3 small dabs on the mirror and quickly (so the caulk doesn`t have a chance to form a skin) place on the mount. Don`t put too much on or the caulk spreads out too much. If you have more than about 20% of the surface covered with caulk then that`s too much.
Three quarters, one in each quadrant, to support the mirror while it`s drying (several days) will be just the ticket.