@zoomkat: Lasers was my original idea, but someone told me I could get away with LEDs instead and it'd be much cheaper. I was skeptical because the light source and the sensors will be about 4' from one another, but he insisted. I guess I'll have to run some tests with both a laser and an LED before I go ahead and buy everything.
I think the below laser modules are the same as I bought in the past. You get 10 for $3.13 including shipping.
You have 36 sensors spread along 48" so they are about 1-1/3" apart. You need to restrict the field of view of the sensors to about 2" at 48". That would be the same view as 0.25" at 6" (divide both sides by so if you put your sensor at the bottom of a 6" hole that is 1/4" in diameter you should get the right field of view. The inside of the hole should be as non-reflective as possible. Putting the LED at the bottom of a similar hole should help avoid interference with adjacent sensors.