Leaks have many causes. Watery output makes them more likely. But poor bag fit or the anatomy around the bag may require modification in the barrier.
True thicker stool is less likely to leak. There are not things that work well placed in the bag but pasta, pancakes and sometimes metamucil carefully can help thicken the output. Lots of caffeinated drinks and alcohol might make things more watery.
I would first worry that there is a problem with the bag fitting. I agree with wearing a belt. An enterostomal nurse might be helpful watching the barrier change technique and usage of flat vs a convex bag or use of a wafer under the barrier.