Spoken like an engineer
The brackets are solid, so I think that the flutter is more from the flexibility of the lexan rather than any counterbalancing underneath. Although this isn't a definitive understanding. The fluttering is minimal, and doesn't affect the air flow. You can extend the lower part as low as you like - personally I'm quite happy with the form it's now.
I still intend on providing instructions on how to make these - it's fairly simple. Gotta find some time.
