If you're buying bucket-muzzles, where the noseband sits is the right place. You need to add 1"-2" clearance, though.
We used one years ago on a laminitic Welsh Section A. Because she wore it all day (she was stabled at night) we originally found the bucket rubbed the end of her nose - and she could get it off. Using a headcollar meant the webbing just lifted the bucket away from the sensitive spot and wrapping the bucket headstall once round the halter headpiece meant it stayed on.
One thing to make sure of is that drain holes are drilled in the bottom of the bucket. They can pick grass through these, but without they can't drink. And I've heard of ponies drowning if they try to because the bucket fills with water.
You can buy web muzzles over here with a rubber base which are a bit more forgiving fit-wise, though they do break more easily.
I have an Irish Draught X who gains weight just looking at grass so you have my sympathy. Cavalrytales Blog