I love fly fishing lakes in July and August. The last half hour before dark is the time when poppers work great. I'd say use a fairly large popper and make a lot of noise on the surface with it. If that doesn't work cast it out, let it sit for a few seconds and twitch it every few seconds and then slowly bring it in by just running the line through your fingers so it makes a very slight wake as it's moving across the surface.
I sometimes hang a soft hackle fly about 18" or so below the popper and on some evenings will catch more on the dropper than on the top water.
Man, I do love fishing ponds on summer evenings.