I think all packaging material should be taxed by weight, but with a multiplier that corresponds to the amount of that type of material which is actually recycled. Glass is heavy, but the data shows that 90% of it is recycled? The tax is 90% off.
This would do three things:
1) Encourage manufacturers to use materials which are actually recycled. 2) Encourage manufacturers to reduce total material. 3) Encourage manufacturers to invest in new methods of recycling to move the multiplier in their favor.