closest I think you will find is what they call a Bicycle Extender bar for mounting lights and so on have one on the mountain bike with two lights on it and its a tuff well made item
link thing
those mounts are primarily for road books. its a bit excessive for a garmin and will leave it vulnerable. RMS rally moto services did stock them but they are out of stock (possibly who you have tried already) rallyraid did have them so would be worth a call to them. there was another company motorallyservices but I cant find a web link for them.
when I made my original road book holder I used 2 old handlebar mounts upside down clamped to the bars with a bit of ally to extend out forward.