Second the dog crate. I use one to tame feral kittens. I put it in the living room and let them watch the complete and utter lack of bad things happening to kitties for a while. It helps.

Oh, also. We’re wandering into territory that raises vet eyebrows. 😉 I usually let scared feral kittens see me fall asleep. I’ve seen skittish cats become besties with people who sleep on my couch. My theory is that falling asleep is one of the least threatening things you can do to a cat.