I saw one once,
a guy ask me if I wanted to go with him to check his trap,I though FISH trap(it was in a farm pond in Ga.) he had built one about 6' square x 3' high. it was on a float ( he had 2x4" on all four sides) with about 1' above the water.2' below the water. it had a funnel from the edger to the center of the sloped to about 2" below the water line. the hole was only anout 5" across.
He had it anchored so it wouldn't move.

he would put corn and peas in the water about the middle of the trap. the ducks would go down to get the feed and when they came up,they would come stright up , they would have to follow the funnel to the top and into the hole.
the hole was small enough that they couldn't find it to get back out.

you could make the trap as big as you wanted. I can tell you one thing, when you walk up to it ; it will scare the crap out of you if your not expecting it.10 or 12 duck trying to take off at the same time, is one heck of a noise.