Sorry to chime in so late on this. I'm happy to hear of other folks fly fishing at Powell. I'm with Ryan, the Clouser Minnow would be my first choice. There's a bunch of pretty flies in TR's box (above) and I believe a few Clouser's. I've caught Walleye close to dusk while targeting stripers. I believe the pattern and time of day fooled them enough to bite.
I almost always use a floating line with long leader and couple more feet of tippet, 0,1, or 2x and let the Clouser sink and then strip that thing in using any number of retrieve types. Feeling that fly line tug out of your hand and setting the hook is about as cool as it gets. Kind of like hand lining with rod assist. Also, when retrieving, point your rod tip directly at the location of your fly, you need to have as much slack out of your line as possible when the hit comes.
Fly fishing Lake Powell won't give you the numbers that other types of faster more robust tackle will yield, but for each fish you hook on a fly the experience and connection with that fish is a truly special reward.
Ryan also mentioned using poppers. One of my favorite memories of topwater with a fly was casting a barbless popper into a boil, it was a very quick, long cast and I was ready. I ended up hooking a nice striper and fighing that thing while the boat was still moving at slightly faster than a slow trolling speed. My friend Hotwheels had an idle problem with the outboard motor and couldn't put the thing in neutral or it would quit. A fellow fisherman saw me and my bright colored fly line working that fish and when it was finally landed he got close enough to yell one of the funniest things I've ever heard. He shouted, "Now I've seen everything, fly fishing a boil at Lake Powell! That's gotta be the most fun a guy could ever have with his clothes on!" We all laughed.
Time to plan some 2019 trips!