Hah, I solved it.
The Kappa latch has a notch in the middle, I Dremeled out the lip of the Givi plate and it held secure.
After a really unpleasant journey this morning (very wet, very windy, lost rear end traction on motorway on-ramp leading to heart-stopping wobble (but all right)) I can confirm that the box is very secure.
I'll get onto the Givi screen later...