Tank bags on this bike may be intrusive: the top of the tank is small, the opening is very close to the handlebar. Bags mounted on a quick lock around it may interfere with the handlebar. Double check fit before buying.
The KTM options are first the 18-litre tank bag (p/n 75012919000 about £139). This is not waterproof, an inner bag supplied.
Another KTM option is the 13-16 litre QuickLock tank bag (p/n 63512919000 about £175). This is not waterproof, rain hood supplied.
Some of the best tank bags for the 790 available by Enduristan. All are fully waterproof by design. For road use with a need for a lot of volume the 4A with its concave bottom is a good fit (13L expandable to 20). For off-road smaller sizes like the 4H (7L) or 4S (7 to 12 L) give more space to the rider. Pics are available for all models here.
The wolfman enduro tank bag has been reported as a good fit as well. Small at 6L, but well designed.