Lock For Screen Door

I'm not sure how much security one can get from a locked screen door, but if the reason is merely to avoid surprise visits and invasion of privacy from friends and relatives, perhaps merely posting a sign requesting folks knock first will do the trick.

Maybe "Knock lest you catch us nekkid" or "Knock to avoid being shot." It might require a serious bawling out for the edjamacation of those clueless jerks who'd just up and poke their head in anyway.