...it's really quite simple. You take a person's most marked physical feature, compare it to genitalia (male, female, or animal), and end with the suffix -bag, -wipe or -muncher.
You can also add an optional reference to excretion from any of the glorious openings offered by the human body.
For everyone's protection, this involved a lot of erring on the side of "medical" rather than "vulgur".
If you're interested and couldn't guess, I just used the frame "Your _ looks like a _'s _." and randomly selected from lists I assembled of relevant items to fill in the blanks. For now, I haven't included the optional excretion reference.