Different bike (2007 Vee) but same problem. It's very frustrating. Every Rotopax solution I've seen so far either requires giving up a case, or is not compatible with my existing luggage setup.
I have one Rotopax fuel jug, of the 1.75 gallon size. I have nowhere to mount the quick-detach handle, so don't use one.
The best I've been able to do is, use Rok straps to secure the can to my Givi side case rack in place of one of my E36 side cases. Leaving a side case home is OK for day trips, but a non-starter for actual travel.
Side-rack mounting is one reason I bought the 1.75 gallon jug. That size has an extra handle on the bottom, through which one may loop rope or straps to secure the can without a proper Rotopax mount.
My top case never comes off, so using a Rotopax mount there is out too.
There does exist an L-bracket that could allow carrying one Rotopax can vertically, in front of my top case. However, it's not compatible with my top case adapter (Happy Trail Monokey adapter plate, similar to this one: https://www.happy-trail.com/givi-su-rack-adapter.html
). No way to connect them.
I don't need the gas can for local trips, and have no place to mount it on real long-distance rides. So mostly, it sits in my garage.
FWIW: What everyone says about the "green" spouts included with the gas cans is true. They're trash. Leaky and dangerous. It's impossible to NOT spill gas with them, way too easy to splash gas where you didn't want it, and in general the "green" spouts are a pain in the ass. Retrofit "water" pour spouts at the earliest opportunity.