In practice, getting the barness stable and fitting nicely, avoiding the brakes and gear cables, is a little more fiddly than one might imagine.
I choose a harness (like the Wildcat Gear Mountain Lion) rather than a waterproof bag (like a Revelate Designs Sweetroll) because it avoids the necessity of seam sealing or using waterproof materials. It's also much more versatile - holding a variety of bags and types of cargo.
I added double-layered elastic straps to jam items for quick access, like gloves and a buff.
I salvaged the majority of the material from a suitcase. The main body is a piece of relatively flexible plastic. I reckon the degree of flexibility is an important consideration - too flexible and it won't provide support - the bag will flop around the place. However, if it's too stiff it won't conform to the load well when it's cinched tight. Additionally, it could fatigue and break.
The rest of the materials are from various places.
Removable spacers. The spacers are sections of a second suitcase, cut into a shape to accommodate the brakes.
Sections of inner tube line the inner of these straps. This provides a bit more grip when cinched against the bars. It's also possible to add a few more layers, due to the compressibility of the rubber, this should put the straps under tension, increasing the stability further.
This lower strap arrangement wraps around a "Crud Fast Fender Front Mud Guard" with the mudguard section removed. This isn't the only way of fulfilling this role, but it's simple and I'm lazy. I'm slightly worried the bag will eventually fatigue the plastic stays, but only time will tell.
I added another length of (hidden) plastic to reinforce the main section. All the holes were made by pressing a tack through the section and then sewing through. I'll probably glue something over the treads on the internal side, for protection from wear and tear. The fabric on the back is pretty unnecessary, but it doesn't add much weight. The edging is more important - preventing chipping/abrasion and softening the edge where the barness meets the dry bag.
At some point I'll add a removable mesh pocket stretched across the front for quick access to a rain jacket and gloves.