After a bit of faffing about, I've settled on this, for the moment, as a way of mounting my old Nav IV on my XCountry. RAM handlebar clamp bolt ball replacing the bolt holding the Barkbuster backbone to the handelbar clamp and a short RAM arm and ball mount onto the Garmin cradle. When young Mr Ghiribizzo does his stuff with the electrics, I'll get it out on the road and add an update on how well it works in practice.
However, for the moment....
However, for the moment....