Unfortunately there are bound to be issues with the mounting of any soft rear luggage on a high level side mounted exhaust *
Forget putting anything under the seat of a 790 ! Squeeze a disc lock in if you careful !poor design of under seat arrangement.
There is a rather nasty looking after-market heat shield available from R&G ? to deflect the hot gases away , this is an option.
Hard luggage as above thread is available but is expensive & bulky when using the bike day to day .
KTM akrapovic fitting means you can have neither so further limit on your options .
Why not try a givi front fuel tank direct mounting bag ? This is what I use from day to day ,a small 5 litre bag & use a bigger 35 for carrying more on longer trips .
Best thing is they are secured directly to tank & don't use scratch magnets & of course you can't lose sight of it ..no problem so far.
Additionally i use a full waterproof 35 litre Oxford bag mounted on rear with a few additional tie down straps for security ( avoiding exhaust gas of course)
Rucksack can be worn as well , but riding on long journeys limits the Fun factor of the 790 when road conditions allow .
But overall the 790 Duke I find more road/track focused & feel the adventure bike would be a better bet for touring .
So anything luggage wise will always be a compromise ...