You are not the first one to ask for advice on a duck pond. The simple facts are,,,,, ducks do create much sediment from their droppings, and water is next to impossible to maintain clear.
I am imagining you want something in the 300 gallon range. You can purchase a plastic liner, and try filtering it. The reality, is your filtration to do a halfway decent job of removing the solids has to be huge. The water will still be somewhat cloudy regardless.
I did run across a successful filtration operation for a duck pond. It was quite many years ago, so cant locate the post readily. It involved multiple 55 gallon barrels, (I think possibly 3) with different stages and types of filtering medium, I think some type of hay was in one of those barrels. and ran constantly with a pretty powerful pump. The pipes were a plumbers dream, so elaborately done. It was all neatly hidden from view.
You can try to get your hands on a large swimming pool filter, and see how it cleans up your kiddie pool after ducks use it. You may be going thru filters like mad.

If you are satisfied with the results, then get a large pond liner and set your filter there.
This one is 270 gallons and $270
If you have an extra $400 laying around, you can try using a sand filter. It will need to be back-flushed often to clear out the solids. I don't think it will make the water very clear from the dissolved poop that is in suspension. Simply put, hard to make not cloudy.
Not sure of your water supply/source , but dumping 300 gallons of water each week, and refilling with fresh clean can get costly. We are on city water, and it is Expen$ive.
I will make an effort to find that tread from years ago.
WISHING YOU BEST,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,and,
