Two somewhat rude examples, for which my apologies in advance.
Make it sons-of-a-gun, since son is the head of the noun phrase (with postmodifier of a gun, in the form of a prepositional phrase in this case). Likewise, mothers-in-law, passers-by, and guests-of-honour.
This one is a classic: ‘I maintain my optimum condition by eating bread with chicken breasts’ (Sunday Times, 13 July 2008).
Since he’s referring to a cut of meat, he ought to have used the uncountable form (chicken breast) — otherwise it sounds like he eats sandwiches stuffed with the mammaries of a fowl (hmm, is this biologically accurate?).
The countable form may also be used when counting the number of pieces, especially in recipes, e.g. 4 turkey breasts.