I use both MetalMule boxes and throw-overs too, but not at the same time

I have a set of Hein Gericke "Pro Sports" and CARGO "Endurance" throw-over cases and they fit very well over the back-seat and down the sides, albeit, resting and connected to the pannier-boxes frame. Be careful using a bungee-cord under the seat from one bag to another as mine have been " cut in two" by the rear-wheel whilst in motion!

The throw-overs save weight and don't look too bad either and using a few bungee-cords to strap them down you'll be okay too