still not off. I haven't followed for a while.
So I used to fly F3b gliders, to do winch testing we would wrap the winch drum with rope or better yet, webbing like truck webbing. the more turns the less chance of slipping. The winch cannot turn, it must be a complete stall for the resistance test.
So do the same, get webbing, wrap it around the pulleys, and keep it tight, rather than grip a jaw which can break, or break the chuck casting, use the whole chuck... if it's a 3 jaw, get a piece of hex and tighten it in the jaws. if 4jaw get a square. That's what you'll put the wrench on... if you have enough wraps it won't move... if it does, get some fan belt dressing for the first couple of wraps, that will provide enough of a grip.