As I was reading this the first thing that came to mind was RFID, but then I read cheaper is better... womp womp. So the next best thing I thought of was a QTI. This one has a daylight filter and it's analog output that you can play with until you find the threshold you like, then set it up at that point like a digital sensor for easy programming.
QTI Sensor ($6USD):
http://www.parallax.com/Store/Sensors/ObjectDetection/tabid/176/CategoryID/51/List/0/SortField/0/Level/a/ProductID/100/Default.aspx
Data sheet at bottom right of page.
EDIT
Beat to the punch by cr0sh... My suggestion is assuming the lexan is clear and there's enough room under it to place sensors. By the way, the QTI is an infrared emitter/detector one a breakout board so we were on the same page.