I saw a pic once of a custom GS mount that went behind the muffler for the left side using Jesse bags like I have on my Funduro. So hopefully someone will chime in with some pics. (I wasn't smart enough to save it.)
I eventually want to fab a similar bracket to use my Jesse bags on both bikes. In my case, the left bag has a hollow cutout on the back side to accomodate the F650 muffler and still mount the cases symmetrically. The hollow part matches the R80 ST muffler closely enough. I just need to design and fab a mount bracket that goes behind the ST exhaust like the pic I saw in order to tuck that left case in close enough.
Mounting equal sized cases symmetrically is a challenge unless you hang the right side way out as far as if there were a muffler over there.
Merritt Island, FL
When a person asks you for advice, they don't want advice. They want corroboration.