I've driven cars that are narrower than a pannier-laden Wee, so being an inveterate lane filterer bags aren't something I want on when commuting. I went for soft luggage because I got an OEM rack and a set of Dri Rider bags secondhand and they are big enough for my purposes. I don't like saddle mounts though as they aren't too secure. So out with the drill, some alloy bar and the universal duct tape.
There is a small cutout in the side panel marked with the yellow circle to allow the belt to hang under the seat lip when under tension. The hole on the OEM rack was already there, but you should be able to find somewhere suitable to bore another if there isn't one available. The straps were folded, hole-punched and heat sealed then brass eyes were inserted. It is fastened with a stainless bolt, stainless washer, eye, washer, frame metal, washer, and a locking nut.
The bottom rings on the bag are held in place with a formed piece of alloy bar so that the ends curve around the edges of the rack and the bend over the central bar is pressed on by the weight of the pannier, stopping it moving. A slight bend in the bar acts like a spring against the rear frame edge and the bar is easily slid into place and clicked over the rear edge. The bar is covered in duct tape to avoid wear - hence the lumpy bits in the middle and ends where there are quite a few wraps.
When the bags are off the front strap hangs out about 2" and the rear about 6" - but can be tucked in between the rear of the seat the the lip of the rack to keep them out of the way.
At some stage I'll make up a 12"x4" flat steel tray for the bottom using the two visible existing holes which can be unscrewed and turned flat against the inside of the pannier mount and screwed into place when the bags aren't on.