PetSafe Come with Me Kitty Harness and Bungee Leash Adjustable, Lightweight Harness for Cats Medium, Lilac

For cats you want something that tightens when they pull; cats will “back out” of a situation when scared, and because of their lithe bodies they will slip through something like this. I would try an infinity harness like this, or the come with me kitty harness. Both will tighten as the cat pulls, but a lot of walking a cat is technique; you always want to walk behind them so if they back up it’s into your legs. You should also practice at home first. Absolutely don’t buy a harness only based on color or price, or you’ll lose your cat the second they spook. Good luck!