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.
This ia a very tiny (hook size 22-24) dun. The pictures are terrible, but it's very fragile and it's one of the first adult mayflies I ever photographed.