Well 1st off yes a plastic stiffener is the way to go inside a Carradice Saddlebag! Yes it works! I have a Carradice Saddlebag
http://kiwipetescyclingsafari.blogspot.co.nz/2011/08/spring-is-in-air.html have a look it worked very well, with My Brooks Flyer and a small support of the seat post.
2nd My Saddlebag (which is off My Africa Bike) is designed off a Carradice Zipped Roll, I had it made for Me as I was after a set of Brown Bags?
http://kiwipetescyclingsafari.blogspot.co.nz/2012/01/bike-bags-made-and-fitted-and-tested.html Yes I did design all the bag's and have them made for me (at a little cost)
3rd ??
"Give us some idea Pete interested to see how you go about making your idea come to life, if the design is spot on the making of the bag should be straight forward, well near enough"Well I'll try too! 1st off why
"PVC 4 sided box" Having the side stiffened with PVC is to hold the shape of the bag and keep my things in place. 2nd
No Bottom (PVC) ? this stop's damage to my gear that is in my bag on dirtroad's. 3rd ?? More to come
draw your bag on paper and cut it out and try it out before having it made for you..
I hope this helps You all about Bags
Pete.......................