This is what I have done for the lid zipper on my Aeronaut. It's called a cobra knot, and if that's not a cool name for something I don't know what is! As an added bonus you can untie it and you have a yard of 2 mm cord that you can use to:
make a clothesline
replace a shoelace
make an emergency belt
lash your bag to an alpaca, vehicle or sturdy friend
make a splint with sticks or a magazine
build an impromptu shelter
build an impromptu pillow-fort
make a tripwire to alert you to intruders
make a hair tie
spice up your love life
hold your seedy hotel door closed
snare rat living in seedy hotel
along with duct tape; repair just about anything
individual strands can be used for thread, floss, fishing line etc...
the list goes on and on..... Add any of your own below!