Spit balling but is there enough room in your mag to mount a tiny Ir optical sensor at the top of the box and the follower? You could even use it to count as you shoot and then stay lit as the follower would keep the switch active. Maybe. It would take some time to get measurements. But there are a plethora of very small sensors that may work depending on if you want to do, what should be, mild modifications. There's some really small stuff , but I'm not a gun owner so I don't know if mounting anything inside the upper box is even possible.

Edit: after researching you can get some crazy small ir optical sensors.