A friend gave me a home made birdbath which had been cast in cement from a plastic household bowl.
first thing I did was buy a plastic bowl of the, same size and cut it down to the same depth of approx 3 inches.
Next I gathered some pebles from around the garden, completely hiding the bowl.
Although its waterproof, it still requires refilling with water occasionally, due to evaporation and bird bathing, I consider this a good thing, as it allows me to add some clean, fresh water to get rid the woodpigeon droppings that can contaminate it.