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The 3/8" in Heat Shrink Tube has a maximum shrinking capacity that is larger than the Mac Charger Wire's Diameter. So to create a proper hold, we need to increase it's diameter in some way. Wrap around it with an electric tape or this sample red wire covering in the picture, to increase it's diameter before shrinking it all the way into place.
Since the end of the charger has a bigger diameter than the wire, it seems to have a nice snug fit. If you feel it is necessary to add more body in the diameter of that part then just cover it with electric tape, etc, before shrinking the heat shrink tube in that area.