OK so one of the strengths of GUE is standards - and here is the standard for light heads.
So...... the standard is a loop - Doesn't say whether it should be bungy or an O ring or cave line. Like many of our standards they are principles whcih are borne out of good diving practice rather than rules.
My own experience is that cave line makes a loop which is hard to snap into one handed, first time, everytime.
Stowing your light should ideally be a one handed operation - not merely becasue it looks cool but also you may be in need of your other hand at the time!
Take the double ender from your D ring, snap it into the loop whilst the light is still on the back of your hand, and then stow on the D ring - using the
same end of the double ender. Make it a quick, slick, practised action which you can do, after practice, without much effort. (don't forget to tuck the cord in your belt - under the long hose - no matter how you clip off your light.
Personally I use a bungy loop tied off and glued then attached to the light in the groove at the end of the ballast which Halcyon have designed in to eth light for this purpose. If you struggle to tie tight line make sure you tie it when wet - line tied dry slackens off when wet.
HTH