I used Mosko moto Scout 25L duffle bag on my previous 690 Enduro's extensively and I will keep using it on new 690 too. It is a great bag, with many useful features (external beavertail / map pocket, internal stiffener, backpack harness...) and it is made to endure anything, like all Mosko stuff.
25 liter size is perfect size for 690 and we designed luggage rack for 2019 690 Enduro accordingly. Here is how this bag fits on 2019 KTM 690 Enduro fitted with our new 2019+ KTM 690 Enduro Luggage rack.
Internal bottom stiffener is great, as base of the bag is slightly larger than the rack. But stiffener keeps bottom perfectly flat.