This one was surprisingly straightforward to identify. The lack of a sclerite at the base of the lateral hump narrows the field quite a bit, and the other options followed fairly obvious characteristics to Clostoeca, which only has one species, Clostoeca disjuncta.
I've been fly fishing amd tying and writing articles and books about both for more than 40 years. My latest book, published by Stackpole in 2007, is DAPPING.