I have a muzzle for Kastle. I started with this:
But he was able to pull it off in a variety of ways (wedge his foot in the bottom part that is so large and also wedge the end in the bars of his crate and then yank it off...he's a jerk).
So then I ordered THIS
one specifically for GSDs but they have other breeds/sizes that are specific for each dog. I really like it and when we are SchH training, I use it weekly. I should mention, the center-forehead strap drives him nutty but it is what is essential for keeping it on his head. He can pant, drink, take treats etc in it.