This is for a hall-effect hi hat controller for electronic drums. The magnet holder is for a 10 x20 x 30 mm neodymiun magnet. The sensor housing can be attached with cable ties (holes provided). The magnet fits snugly in its housing and stays in place through its own magnetic force.

This was deisgned for my own cheap, no-brand hi hat stand, so you may need to adjust dimensions for yours.