I bought this timer from AliExpress for my home gym so I could easily track my rest time without using my phone every couple of minutes. I wanted to mount it directly on my squat rack.

Link to the Aliexpress timer 

The original magnet on the back wasn't strong enough to hold the timer. I also tried using a stronger magnet, but it interfered with the display and the buzzer.

This holder fixes the problem without using glue or making any permanent changes to the timer. It simply slides in from the side and stays securely in place thanks to the internal shape of the holder.

There is also a slot on the back for a magnet (10.6 mm wide and 4 mm deep) that I salvaged from another item.

An opening on the back lets the buzzer sound come through clearly.

STEP file added if you want to modify the magnet slot.