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It's actually floating above the chainstays, and is attached to the 'loam shelf' of the main triangle by a one inch strip of really strong weather resistant double sided tape. The plastic does not slap the chainstays or make any noise. If it fails, it's easy and nearly free to replace.

after reading this thread, i found some plastic sheets at the bottom of some old reusable plastic bags / wine carriers. Cut to shape, used double-sided outdoor gorilla tape, and am pretty please with the result. Photos are attached, I hope (first post). I was concerned about the bearings, so the flap is a wider then the loam shelf, but...