I needed an indicator that would let me know if mail had arrived, long story short, I was waiting for a delivery during a postal strike and the gate unlocking sequence was getting tedious. The indicator started out being a leaf stuck in the side, every time someone put anything in, I would have to do the leaf hunt again.

Once again necessity was the mother of this invention.

Step 1: Suitable Plastic

I was thinking of a bit of plastic with a hook bent into the top edge when it occurred to me that a section of an ice cream tub is just the thing.

I used scissors to cut out a section approx 30mm wide by 65mm long.
Next I cut off a thin strip off the top flange for the L shape which was going to hook into the mailbox lid.
In order for the strip to fall out, I cut two slits into the top to create a spring tab in the middle.
This spring tab has its flange cut off and is bent forward so that it pushes itself off the rim when the box is opened.
I drew a bright red envelope on the plastic with a permanent marker to make it visible from inside my yard, thus saving me from having to unlock 2 gates unnecessarily.
A hot pin makes a neat hole in the plastic tab to attach some thin nylon cord, likewise in the plastic mailbox.

Step 2: In Use

I just slip the indicator onto the side ledge and close the lid. When it releases it is short enough to hang down the side without attracting too much attention blowing around in the wind.

It is quite visible from inside my house ,it does need some head bobbing around to peer through the gate slats, still good exercise I guess.