I'm no lawyer, but I think you're treading dangerously close to something that is illegal. So, imo, the more abstract it is, the better. That way, you can still make sure it looks somewhat like an empty chair but retain your ability to deny that it is (however, please note, maybe not your ability to avoid associated legal fees), if you are subsequently charged with anything???... "Why, no, officer, it's just a piece of random trash that happens to be on my tailgate outside this polling place."
Since we don't see this sort of thing, ever, I assume election officials pretty much have a zero tolerance policy toward anything like it.