This just occurred to me: I was sitting here thinking 'What is as wide as a fan, and has the same dimensions as a fan, and what would also be easy to mount'
Another Fan! (duh!)
Of all of the ideas I've received, I was leaning toward balsa wood--but then it occured to me. Use another fan! If I just dremel out the mechanical parts of a couple of loud old 80mm fans, then presto, perfect shrouds with predrilled mounting holes.
Simple
Bruce