My stinking fistula itches from time to time, too. Not sure but that it's irritated from the drainage. I use some barrier creams and it helps some. I like A&D, Bag Balm, and Critic Aid Clear Moisture barrier from Coloplast. If it gets very bothersome, I do a Sitz bath with Epsom salts, or take a ridiculously hot bath, and soak.
The stupid thing is small, and benign looking from the outside, the little yellow seton is small, but, pardon the pun, it's all a real pain in the a$$.
Your itching could also still be from healing, maybe. You could also try some Lotrimen cream. I did that, too, but found it really didn't work much differently than regular barriers.