If you want to have it hang down more, maybe you should skip the support. I put a large piece of cardboard in my Carradice saddle bag (Nelson Long Flap) to give it more structure. I took a pizza box, cut it to the size I wanted, scored it in lots of places with a pizza cutter wheel so that I could easily bend it and it has worked quite well. I have not had any real downpours, but it has been in some rain and survived.
A bit more detail - if you look at it from the side, think of it as being a big U shape with side panels, I put the cardboard in like that U shape to fit in the bottom, the front (under the saddle) and the back (rear-most part).
I think others have used corroplast plastic sheeting instead of a pizza box, but I have not found a good source for that.
For touring, my Nelson Long Flap it sits on the tops of my panniers, so I do not have it hanging down loosly.
I also have a Pendle model, slightly smaller. I also did the cardboard thing with that one, see photos. It holds shape very nicely this way, no underlying support needed.
