Keyhole mount clip for rectangular tubing

This is an adapter to hang my mailbox from that clips on to my porch railing. The railing is a rectangular tube 38.5mm wide by 19.6mm thick. The mailbox has a keyhole-style hanger with a large diameter of just over 8mm and a small diameter just over 4mm. The back panel of the mailbox is recessed about 13mm from the back edges of the frame and lid.

I printed a pair of adapters and clipped them onto my porch railing, then hung the mailbox from them. It probably would have been better to model t