Note how our model's hands look cramped and arachnoid. If he keeps this up, a finger or two will spontaneously snap and he'll have compound fractures of his finger bones and that will hurt like hell and he'll never play low whistle again. Keep the fingers relaxed. Do not squeeze the tube between the thumbs and fingers. Using the middle joint of each finger will allow the holes to be covered without exerting great pressure, thus avoiding Numb Thumbs.
Some players use the pad of the middle joint of all three fingers of each hand.