I feed my birds wet mash in plastic troughs. As long as the troughs are lower than anything else in the area, they don't perch on them, and they don't poop in them.You aren't worried about their feed mixing with their poops? Have you tried any other feeders, like a bowl instead of a trough?
When you have a large number of birds, i assure you, no bowl smaller than a kiddie pool is big enough.