Plastic is better and cheaper. Plastic is more slippery, so the bale case and knotter friction clamps must be tightened to prevent it from bunching up during needle flyback. Might be time to sharpen or change-out the 60 year old cutoff knives, too. It's not rocket science, just attention to details. The downstream supply chain will appreciate tighter bales, easier to see the twine when unloading and stacking, easier to see the cut strings after feeding, denser bales, less barn space needed and the rot resistance of the packaged and delivered hay.
We don't yet get hay from China or Terra del Fuego, and it can't be 3D printed, so being able to produce your own this coming year is going to save a lot of animals from starvation or slaughter. Around here, even with good weather, cheap fuel, and sprawling subdivisions in former hay fields, horse hay was selling for $6 to $9 per bale, you pick up. Not much left either, only round bales, unless you want to drive 50 miles each way to pick it up in your Prius (put a few extra on the roof). [- Just a side note about my neighbor's predicament !]