I don't know much about the Honda Deauville and looking around on the internet there doesn't seem to be much info on fitting alternative panniers in place of the original integral ones. But I now see why you are trying to get the boxes so close to the bike is so you can pick up the original mounting points.
I would suggest that you'd be better off designing a frame that would pick up the original fixing points and effectively move them outwards from the side of the bike so you can either fit off the peg panniers or something of your own but a much simpler shape. I know this will increase the leverage on the sub-frame and probably mean that you would have to carry less weight. The frames may also be a bit complicated but it much easier to cut and weld a bit of steel tubing than it is to make a complex box in aluminium. Adding a bracing bar across the back of the bike (as you see on Alibaba's bike) does a lot to reduce the twisting force on the rear sub-frame.
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