For the DIY 'pluggers', here's a tip. Get yourself a can of XtraSeal bead sealant, once you've got the plug properly mounted/in the insertion tool, dip it in the XtraSeal, a black gooey quick setting/drying rubber, then insert the plug. Close the can quickly to prevent drying of the contents. The XtraSeal on the plug prevents any tiny air leaks past the plug.
The XtraSeal is also useful for electrical connections. Instead of using electrical tape for over connecting wires, coating the connection gives a waterproof corrosion proof coating.