I know nothing about geese but I do know about building a shallow pond. I sheet of plywood, and 2" x 12" for around the sides, then fiberglass and resin it. One thing we didn't think to do it sink the drain so now I have to sweep the excess water into the drain before I can clean it. We've got a ball valve and drain that goes out into the field behind us.So far, none of these geese have been aggressive to either of us. I was around them every day since we got them at 8 weeks'ish. My wife dubbed me the 'Goose Father' (in a nod to Terry Pratchett) as when they were young and in their 8x16 pen at night and 5x10 tractor during the day, I experimented with 'walking' them and sure enough, they would follow me around the property and then back into the pen every evening when I transitioned them from the tractor back to the pen.
A pic from one such walkabout where I was also acclimating the dog and geese to each other.
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ftr, culling would be a very last resort only if I couldn't rehome (if I decide to try that) and the health and well being of the majority would be best served by doing so. I'll joke about it a lot though.
I think I'm going to build a shallow pond out of wood and use either FlexSeal or one of the the liquid products you use to seal under a wet tile installation....or, I could get crazy and build a wood form and do a fiberglass layup. In either case, it has to have a drain for cleaning.