Clean everything well or make it free of dust and grease, glue with GYSO Polyflex 444, minimum layer thickness 3mm.

But then why such a strong safe? Glued or screwed to a thin sheet of metal doesn't provide any security. Then you can just put the key in the milk crate. That way the safe won't get stolen either.

Hello, using a good double-sided tape would be more effective unless your letterbox is exposed to high temperatures because of the sun. Silicone-type glue could take a long time to dry.

Gyso 444