Contact contamination is likely with plants whose toxins are located on or near the exposed surfaces. This method would most certainly be problematic with plants such as Poison Ivy, as it would be nigh impossible not to contaminate the plastic with the surface toxins these sort of plants naturally exude. Plants who's primary toxicity is due to ingestion might be less risky, but a lot of care would need to be exercised not to expose the plastic to the inner parts of any of the plant. It would be far more desirable to create a reflux chamber whereby the evaporated water the plant exudes would be channeled to a secondary chamber that would isolate any direct contact from the plant material and allow for condensation and collection in a manner physically isolated from the plant material, similar to how a still functions. A field expedient version might be a bit difficult to properly construct, but the theory is sound, and a little "Macgyver" creativity could produce adequate results.