Considering I just purchased one of these evil gizmo's and my Maj hated it I should probably just use it as a pattern and "cut it down to size" so to speak. The one I got was too big but it didn't matter Maj hated it anyway and was going to chew it, himself, and me to death in the process of getting it off.
In all honesty it seems to be very easily made a basic stretchy fabric, some velcro for the closure, and elastic to go over/under the wings. And if you wish a smaller piece of cotton on the inside to catch his droppings.
