Activate automaton behind glass

I was asked to display my work at the Vancouver Airport but the problem was how to activate the Automaton behind glass. Motion sensors do not work behind glass and putting anything on the exterior of the show case was out of the question.

I came across the idea of a light sensor, if it could be mounted inside the case and the light could be blocked then it should work. In the pictures it shows the hand cut out taped to glass but in the actual application we used double-sided tape.