I remember a "letter to the editor" in one of the gun mags, don't remember which one, where the guy writing in said that his neighbor was very anti-2A and even said that if his house was being broken into, he did not want him to assist with firearms. Editor answered by telling him to take a 4x8 sheet of plywood and make just such a sign and put it in his yard. "Just be sure the arrow points to the right house."