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Find the fit
Find out where you camera sits steadily on the jug. Mine has a 'step' in the lens where the inner section goes in and the next one sits very nicely on the opening.
If you're not that lucky use something sharp and 'adjust' the opening until you can get a flat, stable arrangement.
What you're after is figuring out where on your camera the top of the jug is when the camera is sitting nice and steady up against it. If you have to make the hole bigger and have it go right up against the body that's fine. You just need to know exactly where the camera and the jug meet.