I created this to attach my standard US mailbox to the top of a fence post.

The mail box has a small piece of wood on the underside, so this adapter would be fasted to that wood on the underside of the mailbox and then fitted on the fence post.

I squared off the chamfered edges from the top of my fence post for a better fit.

3.56 in. x 3.56 in. are actual 4x4 dimensions ;P.

This model is 92mmx92mm (3.62inx3.62in) on the inside.