Are you using a bag support? Maybe one of those, and secure the straps around the seat rails rather than the loops (to avoid cutting into the straps with misaligned loops)? The straps would be mainly for stabilization, while the weight would be supported mainly by the, um, support.

Mind you, this suggestion is coming from someone who has only used small wedge bags, nowhere near as large as a Carradice Camper, so I am spitballing.