I was inspired by the TinyRM designed by Technik Gegg, so I modeled a sensor like his to fit my printer and resources.  I think this (Technik Gegg's) design is a very clean and well-thought-out design.  I am still modifying my remix to make it better, but as of right now it seems to be working.  the main change I made was rotating the sensor 90 degrees to the moving arm so that I could guarantee when the optical sensor would be tripped. 

side note, the moving arm had to be trimmed slightly after printed to make sure it lined up well with the optical sensor.  I also used M2 threaded insert to hold the cover on.