What I need are the IR LED and sensor (preferably photodiode, but a phototransistor may work). Both should be tiny, and I have to place them really close to each other.

After a bit of research into fiber optics and some other stuff, optical endstops came to mind. I picked the tiniest one and now trying to figure out the best way to put them close to each other. A bit of dremeling, and I have the edge right next to the LED crystal.