Built a couple devices, played, and overall, it works! One issue that came up was difficulty aiming. The IR LED and the lens were not aligned very well on some of the devices, and that got me thinking. I want the IR LED to be held firmly in the proper direction, and I want to do the same thing for the lens. In addition to that, if I can't get them to be precisely aligned, I can have the sights be adjustable. So, if the device actually sends tags a little to the left, you can adjust the sights a little to the left, and the device will be much easier to aim.
With the goal of making it easy to tag other people, I re-designed the front sight and the LED holder, as shown below.
The IR LED is now clamped in place by the acrylic case. To do this, I cut a slot in the acrylic and undersized the hole that the LED slots into. This displaces the acrylic and turns it into a kind of spring to keep the IR LED aligned. The IR LED can then be soldered onto the board while held in the proper orientation.
For the front sight, it now has a slot in it and is secured by a screw, so it can adjust up, down, left or right to align with wherever the device is actually aiming at. While this might not be necessary with the improvements in holding the IR LED in the proper position, the lens can still be knocked around and may not be perfectly aligned. Even when doing my best to keep it straight, the device did tend to send tags slightly to the left (due to the lens being slightly tilted to the left, even after trying to get it to stay straight). With a little adjustment of the sight, I was able to consistently aim at and tag a target.