A handlebar mount isn't impossible - though it would require surgery on the handlebar cover ... probably not a good idea - especially if you change your mind later. Handlebar mounts look clumsy (IMO) anyway.
I bought an accessory shelf ( see http://www.adventurebmw.com/eshoppro...LF_K1200LT.htm
) to allow me to fit a Street Pilot 2720 and modded it as follows:
I didn't like the wide deep flat shelf provided, so I removed that and fitted an aluminium bracket (inverted 'L' ) which hangs down (slightly) over the top of the instrument surround
and fastens to the bolts (1) (which were replaced with slightly longer bolts.)
Originally (other GPS) I used a Touratech mount but (at the moment) I have a Ram-mount ball fitted to the front face of the bracket and the Garmin bike mount is attached to that.
I drilled a hole to the left of the light at the top of the panel, fitted a rubber grommet and routed the cables through that. My bike doesn't have a BMW intercom, so the audio from the GPS is fed (using an isolation transformer) into my Eurocom. - I would like to be able to feed it in as an 'override signal' to the radio installation (but I haven't worked out how to do that yet.)
It shouldn't be too difficult to fabricate a bracket from aluminium. - You'd need 2 vertical strips (well, 'leaning backwards' strips) to run from the mounting bolts, with an L-shaped strip running between them.
NB: It's a good idea to put some sort of padding between the bracket and the bike - a couple of turns of self-amalgamating tape around the 'vertical' arms would do and a felt disk between the L-shaped plate and the tupperware. - Otherwise, the attachment will wear the plastic a bit.
I had a Touratech mount for my Garmin 2720s and I was very pleased with it. - It was far better than the Garmin offering. Unfortunately, Touratech don't seem to do an equivalent for the Zumo.
Sorry - I can't provide a photo of the installation; my K12 went into storage yesterday and I won't see it until Spring.