Very timely. I'm looking to do the same kind of thing since it looks like it might be a few weeks until Motech gets its act together with the top rack. I ordered the Givi universal mounting plate for monokey cases (which I use successfully on my 1050 Tiger) and I will try to see if I can mount it directly as you have, or if that doesn't look doable, to have a plate fabricated to put the Givi mount onto which then bolts onto the bike using the 4 mount points that are already there.
It doesn't seem like the stock plastic rack can really take any significant weight since there's not really anything underneath it. Are you mounting it using the stock holes drilled into the rear subframe or are you just mounting it to the stock rear rack?