Effectively what you need to create is a cross bar across your yokes. There are several mounting kits that you can then use as the majority of these are fitted to BMW GS models with trial style handlebars.

Or try this simple thing that just mounts to your ignition lock fittings, you'd just need to screw a Garmin mount to it.

Looks cheap as chips to me, and that's what I will be trying with my Garmin unit on my ST

Here's my mounting for my GPS3+.

and again

where did Jools & Rockhopper get their mounts from ?

Mine is a Garmin one.

Pictures of the Touratech Mount
 

Here is my system with the sat nav in place

Touratech Mount
 

And one without the Quest

Last one
 

This one lets you see the two brackets and was taken looking back from inside the screen.