I started with the schematic from the Gen 7 end-stop and went from there. My biggest problem with the schematic is that the output signal (probably) will not give a 5v high output, according to my math the output will range from 0- 3.7v. I'm sot sure what a digital input pin on an arduino will accept as a high but I would rather the output could go the full 5 volts.
I used the same photo-interrupter TCST2103 and corresponding current limiting resistor as well as the pull up resistor for the photo-transistor.
It needed to have an led for debug purposes but I couldn't decide on the final wiring, so I left 3 different component landings near the end of the board. The idea is for the builder to use whatever landings they feel necessary: led on all the time, led on when the sensor is blocked and led on when the sensor is not blocked. Use the 2nd one with caution because it will also have an output high of 3.7 volts.
The board was carefully sized to fit onto a Prusa End-stop holder with minimal overhang. The connector is placed at the end of the board so the wires can be routed out of the way.
The board (3 ordered from http://oshpark.com/
for around $5)
blue led (surface mount 0805)
180 ohm resistor (surface mount 0805)
2.2k ohm resistor (surface mount 0805)
2 - 1k ohm resistor (surface mount 0805)
Plus a complete assembly of a ~.1" pitch male/female connector (don't forget pins), or solder wires directly to the board. This project was designed to use a 2.5mm pitch JST connector.
Mounting Parts(assuming mounting to Prusa):
Printed end-stop holder
2 - M3X18 screws or similar
3 - M3 nuts
2 - non conductive spacers or small diameter washers to space the board from the printed holder