I would not recommend soft panniers as they will rub against the bodywork, and leave scratches, unless you build a frame to prevent that. As far as I am aware there are no off-the-shelf soft pannier frames available from any supplier.
As far as opening the fuel tank lid, you would need to lift up the pannier each time you want to refuel (and design the frame in such a way that it did not impede opening).
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