You can build a raised pond by setting a plastic or fibreglass mould on a layer of soft sand, and building up round it using rocks and/or bricks. To prevent it from cracking when full, make sure that the whole of the base is supported.
As easier alternative is to use a wooden half-barrel, lined with butyl, or a plastic water butt cut in two.
Make sure that small animals can get in and out, by giving them a 'ladder' of bricks and/or planters inside the pool and, outside, either by providing another similar ladder or by siting the container against a raised bed or low wall.
Choose plants with care: most native plants are too vigorous. Small plants like bog arum Calla palustris or brooklime Veronica beccabunga will be entirely in scale, while flowering rush Butomus umbellatus will fit but dominate. Among the exotics, pygmy water lilies such as Nymphaea pygmaea 'Helvola' or 'Alba' are fine, and also the dwarf Japanese reedmace Typha minima.