I used the zip ties for about a week. I hated them! The ducklings grew so fast, I had to replace them every other day. The ties have to be snug enough that they don't just slide down over the foot, but loose enough that they don't cut off circulation, and you must cut them off and replace them while there is still enough of a gap to get the scissors down in without cutting the leg open. This was a totally traumatic experience for my ducks, and I was in constant fear of slicing a leg each time I had to change them
Someone on here recommended velcro rings. I think this is a better idea, though I've not been able to find the kind of velcro I need in order to give it a try.
Someone else mentioned yarn for the short-term until they are grown and can get permanent leg bands. I think that might work well, too.