In order to mount the transformer right you need to read carefully the data on the label indicating the colors of the terminals for primary and secondary windings.
ATTENTION! Under no circumstances should both ends of the fixing bolt be allowed to simultaneously come into contact with metal chassis or framework so that an electrical path is formed through the screw in the center of the transformer via the external framework.
This would constitute a short turn and could cause irreparable damage for the transformer.
The easiest way to mount a transformer horizontally is the one presented in the figure below, using a metal plate, 2 rubber discs, and a clamping bolt.
For any equipment, you can choose the mounting system that best fits your needs.