Creating signs sounds pretty fun. I actually tried looking for some protest sign generators, and found a couple but none were that good. I also can't think of any messages that are as good as the ones mentioned.
I guess as long as you keep in mind that the WBC members aren't likely to change their minds and use the signs instead to communicate to the rest of the public that they're ridiculous, it should be a successful counter-protest. Try to take some pics!