We use them routinely in wet / cold weather; as others have said, they burst off in a few days. You can get large and standard sizes; some of our large singles need the large but the standard really seem to be big enough for almost all. Some very tiny lambs need a much smaller one - not sure if you can get them; our local agri merchant doesn't have them.
We now only use the clear ones, having had a handful of cases of very aggressive rejection of a lamb wearing an orange one. I would be concerned about the same reaction to a home-made-from-carrier-bag solution, although I applaud the idea.

One of the very aggressive ewes beat up (and
really beat up) another ewe's lamb wearing an orange mac, which was the point at which we abandoned using them, since carefully ensuring that mum is happy with orange-macced baby was not, apparently, protection for it from being beaten up.
We do get the odd ma who won't accept her baby in a clear one, but so far haven't had a ewe trying to kill someone else's lamb wearing one.