In reply to Cbarrasford:
I've used the Gecko 20 on various things and my personal findings are:
It fits nearly everything you could dream of in it but due to the nature of the structure of the pack it doesn't always hold it too well with the bottom of the bag often sagging below and swinging a bit with heavier loads (maybe the cause of your issue) - make sure you tighten the cord bits by your shoulders up as this is basically the backbone of the bag.
Whilst using it for shorter runs (as my other bag was off the run) I found that without it being nearly fully packed out as its really slack and moves far too much for it to be practical.
The fitting across the chest needs to be cranked right up in order to stop it moving excessively and the elastics slip back through the buckles if not tied off - I've added extra elastics and crossed them on mine to try and counter this a bit.
Its not very customisable...I have a salomon pole quiver on mine which is beautifully attached using 3 lengths of cord as there's no really practical attachment provided.
The strategy I've seen people using is a waist pack under the bag which can almost act as a waist belt and provide some support under the bag but I don't know what your bag strategy is
