What materials can be used to stiffen a leather bag? I'm early in the process of making a leather doctor's bag (also known as a Gladstone bag), in this kind of shape:



I've bought a side of ~2mm cowhide for the body, but I know from past experience that it's too flexible to stay in shape while standing on the floor without something to stiffen it.

So, what materials can I use to stiffen the leather in the straight part of the bag's sides?

I've considered the following:


Card (as used here) - this worries me because it will degrade very quickly when wet.
Paperboard (chipboard in the US, as used here) - similar issues to card
Interfacing - I've never seen this used for leather goods but it's a plausible material
Sheet plastic - this is lightweight and waterproof
Sheet metal (probably aluminium) - strong, but probably not flexible enough and heavy
Steel framing (rather than a full sheet) - this seems to be what hard briefcases use for strength but is almost certainly too stiff.


I'm looking for something that will enable the bag to stand upright without sagging, but I'm making a bag not a box, so it needs to retain some flexibility.
One thing not on your list is steel wire.

This is a sort of steel framing I suppose. But it can do a lot while remaining light and flexible. I would think you want something in spring temper, so perhaps piano wire of sufficient gauge. I suppose whenever I've seen these it's been mild steel though, somewhere around .050-.100" diameter.

"U"-shaped members on each side and each end might do the trick. They could be sewn into seams or otherwise concealed.

Interestingly, these members resist torsion (twisting) as well as bending. So, for example, if you wanted a bag that didn't flop down but was free to pull apart for access at the top (and pull together at the top to close), you could place a square wire frame at the bottom and a U or square frame on the sides only (not the ends). The frame would tend to make each side "hinge" in and out as one, roughly like the cover of a book, rather than having to pull each end separately to open.