My first UKs had "floating" waist bands - they required a belt to keep them from slipping down your hips, but they accommodated day-to-day variations in girth. Newer ones come with a set of snaps on the waist band.

As to your original question, I think most UKs sold that I've seen are too big for the person in them (if not worn too low) and they can end up looking blousy around the butt. Because I was able to try it on in person, rather than order over the internet, the last mocker that I bought was a size smaller than I usually get, and I like the fit and shape much better.

Regards,
Rex.