Since you are fishing your lunch break, I doubt the dawn and dusk options fit. You can try to bright day - bright fly and dark day - dark fly bit. With nymphs I have only had luck, under these conditions, when the nymph is on or very near the bottom. A small, quickly stripped streamer might work. Poppers might work. Big parachutes that imitate wolf spiders, etc... might work if fished with more gentle movement.

Good luck and please let us know.

Ed