Chris
I knew you'd find a site ....

I had every confidence in you

This is from your link
http://www.karlsims.com/harmonograph/We made one end 'swingable' in both directions, ie, a 'sperical' bearing, so you can do it with 2 pendulums.
Weights were 2 litre plastic bottles filled with wet soft builders sand, circa 4-5kg, the pen arm hinge was one of the 'polypropylene?' aileron hinges used on model aircraft.
Try an adapter to put more then one pen on, in different colours. Cute patterns.
I seem to remember spending a somewhat sozzled Saturday afternoon in Steve's garage, playing with the thing when it was up and running.
I never really grew up

Fortunately ..
Dave BC