Here’s a prototype of the arm. The arm is PVC and the box is 1×3.

In this photo you can see I’m using wooden dowels for the stops. I used a wine cork (with a dowel in the center) for the upper limit stop, so that it won’t be noisy when the spring returns the arm.

In this photo you can see the unit upright. The spring returns the arm to the up position. The cam assembly (yet to be built — I might use one of my old props) will push from the top (against the spring) to move the arm down.

The pipe pivots via a hole drilled in it, with a dowel as an axle.