I think the easiest way to produce natural hybrids besides putting all males of one species and all females of another species together is through nest crowding. If your pond has very minimal spawning habitat for sunfish, all the sunfish in your pond will be competing for that much less real estate. This can cause nests to be built that much closer together. IME with RES, they tend to nest on the edge of BG nest colonies. If they get jammed that much closer together, some milt from a male is going to drift over the eggs of a different species of sunfish. There are other factors involved as well, more turbid conditions can lead to mates having improper identification being one of them.
