Here is my cheap'o one i made a lil while ago.
This one is too confrontational. More like an ultimatum. The one the OP posted says the exact same thing but isn't confrontational and doesn't sound like an ultimatum. It is simple, to the point, and educational. It says, very simply, that the potential armed robber isn't likely to oblige their no firearms policy, but that permit holders will --and the trade off to disallowing them, the ones that are willing to oblige, is to simply shop elsewhere. The bottom line, "how much do you know about your other shoppers" is great.
Seldom does short, educational, non-confrontational and efficient idea to change policies regarding this come along. I think it is genius, and the fact it costs nothing more than printer ink and paper is great.
Perhaps making an alternative sign for them to post would be a good idea too, but everyone would have to get on the same page. You could hand out the index card or whatever with the OP's message (the color one works better in my opinion) and along with it, an alternative sign. You should find and join all your local shooting sites and seed this as much as possible, get everyone's opinion in your state, and settle on an alternative no-firearms *except* sign.
I don't know what kind of sign you'd need, I think offering one would increase the chance of them actually changing their policy, in particular with small businesses that don't have these decisions made ten states away in some board room. But the OP's idea is ingenious, and in combination with alternative signs to post in the window, I think you could make some change. Once one starts, they will all follow suit, they want the money more than they care about you carrying or not. I can tell you for a fact that Starbucks will be a good place to start. They are carry friendly, very carry friendly here in their home state. Big deal about my local one on Opencarry.org and the resulting good that came out of it (Starbucks stood behind folks legally carrying and the cops got educated as to what the law says and were told not to harrass folks when some nut calls saying "I see a gun!")
I'm lucky in that I live in a state where a business can't prevent me from carrying. Only bars, big outdoor concerts (?) and schools are off limits (but you can carry if dropping off or picking up) local and state buildings with restricted areas must provide a check-in, and federal buildings, they are off limits. The city of Seattle tried to make all city property, which includes all the parks and a lot of other real-estate into no firearms zones. Since it violated our very simple state law, it fell through and wasted money just like everyone told the mayor it would.
When it comes to no firearms stickers around here, unless in those places above (which I rarely frequent save the VA hospital and post office) I just ignore them. They are rare outside federal buildings here anyway. I conceal carry, but I'm not too fastidious about keeping it from showing if the wind blows my shirt or jacket open, or if it prints if I stoop down. If I'm in an anti establishment for some reason, the worst they can do is ask me to leave, and if I refuse, I can get charged with misdemeanor trespassing. Never been in this situtation though, and I've been here over ten years. If I were asked to leave, I would, never come back, and also post about it on all my local sites.
But perhaps having that card would be a good idea here too as then they'd know why all of a sudden business dropped off.
Great idea, OP, just great. You really need to go onto your local shooting sites and, oh yeah, go to your local shooting clubs. Not just ranges, the clubs. They'll really be into it. Gun shops and such might be a good idea to advertise the idea and hand out cards too.