Congrats! Seems like a neat project. I just skimmed through the article and saw "it tries to be cheap". Maybe you can evaluate whether ultra-cheap steppers can give you acceptable results. They cost around 1/10 compared to the kind you're using. They are a lot slower and have less torque, but probably could give similar results.
Sorry forgot to mention I'm thinking specifically of the 28byj-48 motors. I had an (unfinished) project making a motorized water sprinkler for watering lawns in non circular shapes, and these motors were up to the task. If you can achieve your goal with them, you'll probably shave a nice chunk of the cost (not only on the motors but also on the drivers).
Cheers
Good luck and thanks for sharing this!