Regarding fixing Kriega luggage packs to the pillion seat area of the Duke, I used some Kriega webbing loops like this :-
The above hypalon webbing loops are 95mm long whereas mine are about 60mm long. I cannot remember the exact model of bike but I think it was a Ducati model.
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I have successfully used these links on two other bikes. You just need to remove the four rear trim panel bolts from under the seat and fit longer bolts, packed to suit with spacers or washers.
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As you can see, the loops are screwed to the frame and then the normal Kriega straps loop into them.
My concern regarding fastening luggage to the pillion seat is that the security of the assembly is only as strong as its weakest link, which is probably the single rear seat latch. By using loops screwed to the bike frame the luggage is more secure. I carry a Kriega US10 all the time, containing puncture repair and first aid stuff. When travelling further, I double up with a Kriega US30.