Sorry, plastic does not block UV rays.

It depends on the plastic. Polycarbonate blocks UV fairly well. PET would be the choice for any type of solar water-treater.

It's my understanding that Giardia are not particularly affected by UV-A or UV-B, but would require UV-C radiation, which I think the Steri-pen must generate. There isn't much UV-C in sunlight by the time it reaches the ground.