I have a pair of panties that doesn’t tuck, but actually uses them. They are a g-string style, with an extra piece of elastic running vertically, The elastic splits your package (making it seem like two separate parts), and holds the “ovaries” up against you; they make it look like you have female genitalia. The front acts like a gaff, being tight. You can even wear this under a bathing suit (which I have done) or tight pants.
They come from a company called Koalaswim (despite the name, they’re based in the U.S.) and they make several different gender bending panties, one of which they say can be worn under a bathing suit.