Carradice Pendle: Stiffening Base

[Wanderer]

I've got a Carradice Pendle on its way from SJS Cycles (would have been here already, but for the snow). I'm going to use a Minoura Space Grip on the seat post to support the bag in a levellish position. I want to put a lower dowel inside to stop the back of the bag sagging, with something lining the bottom to stiffen the base.

Like someone did on this Barley:



and like Acorn Bags do with their Large bag:



My question is: has anyone got any idea what sort of material I should use for the base stiffener, and where can I get it from?

[Wanderer]

Well, it seems that some people use a corrugated plastic sheet, called Correx.  It's used for signs (estate agents' signs for example), among other things.  I've found some on eBay in small enough sizes and quantities, so I'll give it a shot.