Hello
I have the same problem i dont put luggage on my storm that often but after i had put on hi level cans (no foot pegs etc) i was due for the annual trip the the isle of man so something had to be sorted! after standing in the garage looking round for ideas i tried a stanley mitre block, yes i know bet you think what the F**k but let me explain the mirte block (i am tight and only wanted to use what was lying about

) is about a foot long and six inches high and it just fits on the pillion part of the seat, cover off obviously i put it on so the base of the mitre block faced up and ratchet strapped it to the seat very firmly/tightly (took the seat off to do this) and then put the seat back on shortened the straps between the panniers as much as possible (i have oxford expanding sports panniers) and put them on to the mitre block and then strapped them to that it works ok and keeps the panniers away from the end cans. My panniers were full and i also had a large bag on top and had that tied on so i could brace the luggage against my back under heavy braking.
Its not ideal but it worked ok 90 mph + and numerous rounabouts it is a bit wobbly because its fastened to the seat but its what i am going to do again this year might not be suitable for touring around ( i only have a short trip to the ferry)but something similar might work. If you have a mitre block you could always try it

and see what ya think.
Hope my ramblings have given you some ideas.
See ya
