By far the easiest thing for sanitation is an aboveground container, like a kiddie pool or whatever, because remember you are going to have to be dumping or draining the water out of this thing on a FREQUENT (potentially daily) basis. Ducks are FILTHY.
"Dig hole with shovel" only works if you are on make-bricks-with-it clay, or have an essentially infinite free source of water to add... and becomes nearly impossible to keep clean with ducks. I dont' have ducks myself but have been around them, and have worked with small ponds and pond containers a *lot* professionally, and really, I have to say, go with the kiddie pool or equivalent (!).
Good lluck, have fun,
Pat