Zip ties will eventually break. And I'd never recommend wind slits. I've done this before with stainless bands (Flags USA) and stainless steel brackets (Grimco has them). (I've mounted .080 parking signs this way--I didn't want to drill into metal posts.) I didn't want to pay for a banding tool, so I got the "slotted" steel bands (like radiator hose clamps). Then I screwed a screw eye into each bracket and hooked up heavy duty bungee cords to keep the banner taut and absorb wind gusts. I also used some wire (with plenty of slack) in case a bungee failed (but it never happened) so a loose corner flapping around wouldn't shred the banner. The key is to sew some nylon webbing with an O- or D-ring in each corner. Also, you have to allow space for the bungee cords. A 14' banner between 2 posts 14' apart doesn't work. Make the banner 11' long. If the bungees are too long tie some knots in them. Actually, you can also get away with mounting bungees to 1 side only.