The weather has turned too cold to cast a rotor, the polymer binder won't set.

Here's the concept of the magnetic counter balance with an exponential spring rate.

https://linmot.com/products/magspring/
https://linmot.com/blog/floating-weights-with-magspring/
Cadman, have you seen these magnetic springs before. If so what are your thoughts on them.

No I haven't.

But these won't work for this experiment anyway. They have a constant force over the entire travel. I want an exponential force.

Besides, why buy one when making one is so easy?