A pattern called a Woody has worked for me in the smaller lakes in YNP and the private ranch ponds. Real easy the way I do it:
Hook: #16 1xl nymph (Dai-Riki #060).
Thread: 8/0 black.
Tail: lemon woodduck.
Body: peacock.
Rib: pearl midge flash.
Wing/hackle: lemon woodduck over top 180 degrees of hook.
Basic unweighted Prince nymphs fill the same niche.
I never visit any lake anywhere without BLM Nymphs in PMD and Brown-Olive. My largest rainbow ever was taken on the brown-olive in Trout Lake in 2009. My one regret was that fish was the only one I caught that day that I caught fishing blind rather than sight-casting.
Owner, Yellowstone Country Fly Fishing
Head Guide, Parks' Fly Shop
Fly Designer, Montana Fly Company
Author, Yellowstone Country Flies and River Characters