I have one at our cottage that I built. Pics follow.
It's about 5' x 5' and uses hoses to hook up to hot and cold faucets plumbed from the cottage. We are on sand so drainage is not an issue. I have six grandsons between 16 and 21 and they are required to use this rather than any of the showers inside. It works really well.
PM me if you want my plans and BOM.