It does look like a giant footprint ! ❤️
and the bamboo "waterfall" feature is awesome.
alternative to a plastic liner and even cheaper (!) is clay.
also, padding the hole with lots and lots of organic material will eventually rot away and water proof the pond.
another, is the use of bentonite. not sure how cheap it is tough.
I made a similar pond but there is no room for me. only frogs and water plants. the frogs sing all night long. I doubt I could now sleep w/o them 😊
one last thing. the best for slug control is a duck or two 💪
regards from Portugal.