To check the width of the mouth is pretty easily done and you don't need to buy any gizmo or gadget to do this with. I improvized. Find a peice of string about baling twine size and tie one end to the center of a pencil (not sharpened). Gently draw the twine through the mouth using the pencil to lightly brace against the cheek of the opposite side. Grasp the twine beside the cheek/lip on the non pencil side with your fingers and tie a knot in the twine at that spot or mark it with a marker. Remove the twine from the mouth then measure the distance between the pencil and the knot or mark and this will give you an idea of how wide the mouth is.
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