I took a couple photos to explain the corroplast stiffener better. The corroplast is a white rectangle in the photo, U shaped so it it up against the saddle, in the bottom and comes up on the back side. Sorry the first photo is out of focus. This is the Nelson Longflap. The cotton canvas is pretty stiff, but when you put some weight in the bag it will sag without some extra stiffener, thus I added the corroplast. As I noted above, I used a pizza cutter first to crease the corroplast where I wanted to bend it, that helped in keeping the bend to the exact place where I wanted it bent. It is essentially a form of cardboard, but made of plastic. And being corrugated, it bends and folds best in one direction. The second photo shows a couple of the creases pretty well.
Another cyclist once saw my bag on my bike, we were both riding at the time so he could not easily look at the bag from all angles, and he asked if I had a rack under it. I said no and asked why he thought I had a rack, he was commenting on the lack of sag in the middle of the bag. That essentially really explains what the corroplast sheet accomplishes, but that is all it accomplishes.
Carradice puts an extra layer of fabric inside as a liner, my two Carradice bags I have have different liner fabrics, the bag that Nun has in one of his photos above also has a different liner fabric.
You can see the extra length of the flap is folded and will be under the flap when the flap is closed, the extra length of flap is held in place with a couple snaps when not in use.
I have thought about getting out some needle and thread to add two D rings to the bag so I could add a shoulder strap, I might still do that some day.
There are some quick release systems out there too. It might take me five minutes in the morning to run the leather straps through my saddle spring loops to attach it when touring. I have thought of a few other options and might get around to finishing one of the projects I started.
I am in early stages of planning a tour this summer, and this thread has gotten me to thinking i might have to use the Nelson Longflap on the next trip.