This year was apparently "Send Evan a Musical Birthday Card Year" which i thought was great, and consequently i hadn't thrown out any of the musical cards (from August). I cut the music mechanism (chip, battery, speaker, switch) out of a card and came in early today. Crawled under Chandler's desk and taped it to the underside of his desk such that when he closed his drawer, it would squeeze down the metal switch to complete the circuit and play "Car Wash."
I figured it would take him about 5-10min to figure it out. It took him more like 2 hours, during which he walked around our side of the floor complaining and looking really confused- apparently it kept cutting off at the right times so he couldn't quite locate the source of the music. It helped that i'd told a couple of his labmates and they played along. Eventually, he took a screwdriver to his desk and had it partway dismantled before finding the thing. Worked out even better than i could have hoped.