For use with either openXsensor or grmis $5-solution.
For the optical part, I ended up using a photo transistor.

The one I used is this: https://www.elfa.se/en/ir-phototrans...p3c/p/17522735

This will only work well in daylight, but that's all I'm interested in.

I suggest using a 10-20 kOhm potentiometer to determine a good resistor value.

The signal line will go high when the phototransistor is blocked from light.

Signal goes to Promini pin D8 with openXsensor.
Signal goes to Promini pin D4 with grmis code.

Schematic below: