simple pond for box turtles?

I have an enclosure in my yard with several box turtles. I want to
upgrade from water dishes to a small pondish setup.

I have approx a 5'x5' area to work with. Box turtles like to soak, not
swim, so it shouldn't be too deep, at least in some places. They also
like to defecate in the water, so it should have a filter and a
reasonable volume. I am on a limited budget.

I have a few ideas. I was thinking of using a tarp or plastic sheet
(drop cloth, shower curtain?) as a simple liner, with a circulation
pump to a small veggie filter filter in a raisedbin. A pre-filter on
the pump, perhaps an overflow onto lava rocks or screen, would catch
feces.

Any thoughts?