There are various mirror fittings available, but they are often both ugly and ineffective.

These are good ones. The unit consists of a ring head, washer, steel bolt (2 5/8" long by 9/32" in diameter) and plated insert (7/16"), all in a light antique finish. The bolt can be docked to suit your frame width if required. 3 7/8" long overall.

There are two units in a set.