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The Pulla Bung needs to fit closely to the end of the section so that the tube sticks out. First push it in as far as it will go.
Keep your nail on the bung and take it out. Mark where this is. It is important that you are accurate.
Cut through the bung one rib bigger than you think you need. You can then trim one rib at a time to achieve a perfect fit.
Attach your connector to the other end of the elastic and thread the elastic through the top kit and Pulla Bung.
Thread the elastic through the base sleeve and tie a knot. If you need more tension push the sleeve up and tie another knot.