Fittings to use regular handlebar light clamps
Screw into low rider boss (or use a bit of broom handle with ahole drilled through a long M5 bolt and a couple of washers).
Clamp to fork blade (use a boss if possible as a clamped light could get into the spokes)
Fork crown (Fork crown brackets do require space below the bar bag, which may not be available on a small bike.)
Fork crown and fork blade are the only reliable locations.
A fitting to periscope over the top of the bag whould need to be fairly tall, and you could well end up with something that seemed OK which didn't work once you'd put stuff in or on the bar bag.
A light attached to the bar bag would tend to bounce around and be very offputting to use.