Two top boxes..
The frame is all flatbar, some of it welded together, but really theres no reason why it couldn't be done all with bolts and carefull bending in a vice. A power saw and a pillar drill make it less hard work than a hacksaw an a drill.
The (top) top box is fiberglass so weighs almost nothing, its empty till i get to uni and then its where i stash my elmit.
The tube has either spare petrol or oil, or waterproofs.
£6 for the steel box (which has been laying arround for a couple of years)
£2.50ish for the amount of flat bar used.
£2 nuts and bolts
£2 lock and hasp
Had the topbox on my last 3bikes and the tube was free.
Wouldn't take long to add brackets for panniers after this, but being a 125 its going to struggle as it is when its loaded up!
Doesn't seem to have affected the aerodynamics