Two holes in a horizontal board - drop dowels into each one to act as pendulums. The pivot mechanism for each pendulum was one of those trapezoidal box cutter blades. (In the above illustration I suspect they are nails protruding through a cross-piece.)
Weights are anything you can find. As long as you can move them up and down the dowels to change the period of the pendulums.
A clipboard screwed and glued to the end of one pendulum. The other pendulum has a hinge with the arm holding a pen.
