Having fitted the BMW Luggage Plate in place of the pillion seat, my top box now clips into that and I can use the pillion back rest on the top box...at a push. Since I'm a relatively normal sized human being rather than being grossly obese, I need to slide quite a long way back on the seat to make contact between my back, back protector and jacket and the back rest.
The alternative would be a Corbin seat with the optional slot-in backrest. In either case you'll either need to have enough flexibility in your hips to hurdle over the seat or stand on the footpeg and fold up your lower leg when mounting the bike.
HTH