Anyone done this and used successfully off-road?
Look at Monkii Cages.
http://www.cyclemiles.co.uk/shop/brompton-gifts/monkii-cage-bicycle-bottle-cage/#.WJMoaVOLTcs
The Gorilla clip / cage is more like salsa anything
http://www.cyclemiles.co.uk/shop/brompton-gifts/gorilla-clip-for-gorilla-cage-and-monkii-system/
Nearly £50 for a set though...
https://www.amazon.co.uk/d/Bottle-Cages/Zefal-Gizmo-Universal-Bottle-Mount/B007VIDC7C
Bit of a pig to fit, worked ok for light loads and a blackburn cargo cage
Thanks, that Gorilla clip might be the way to go and only need one side. Already have the Anything cage but not the forks they used to go on!
Blackburn outpost cages have slots so you can use jubilee clips to attach them (as well as holes for bolts/bosses). Just make sure to wrap some old inner tube around the forks first, and if they are suspension forks, don't clamp them too tight!