You can buy the part for the Klick Fix handle bar bag that screws or rivets
to the various bags of makers that use the Klick fix parts.

I have an older Ortlieb Camera bag.
I use a KF wire support , that fits in the loops on the sides for the D rings ,
and secure it with the waist belt fitting slots on the back.
a short section of webbing and a tri-glide Fastex piece[x2] does the trick.
It's not a Hard case ,
But the bag closure is a roll top, so drier than the flap lid
on their regular Handlebar bag..