Reinforcing the mouth of a large vegtan gusset bag

[Mallethead]

I'm designing a large (around 40 by 30 by 7 cm) gusset bag from 2 mm vegetable tanned leather. I'm using a three-piece gusset design for added rigidity. I know from experience that large bags like these will eventually start to become rounded at the top as the bag starts to form around your body.I would like to prevent this as much as possible, keeping the box-like form as long as possible.

The best solution I have thought of thus far is making the lid a separate piece and hiding a slat of 2 mm PP sheet inside the joint between the back piece and the lid. I wish I had some wooden blinds, I could steal one of the slats and use that. I also considered adding a small formed pocket on the inside, but this would make the narrow bag less practical.

Any thoughs or recommendations?

[bikermutt07]

I'm not sure I know what you're talking about exactly, but..... Home Depot and Lowe's both sell narrow slats of different wood for projects. You can get some of them as thin as a ¼"x¾".

[Tugadude]

A wooden yardstick could also be cut down to size.

[alpha2]

How about a paint stir-stick, usually thinner than a yardstick and about the same otherwise. Although, if you use wood, and it gets broken during use, that will be worse than not using anything. I like your 2mm PP idea the best. 

[Mallethead]

Thank you for your comments, I think I'll probably go ahead with the polypropylene. Of course, I'd like to keep the materials as natural as possible, but I think this will work out OK. It's tough, flexes a bit and won't break.

I'll probably mold the leather around the slat, but won't moisten the entire lid piece. If there are any stains, they'll be on the back anyway. I'll treat the leather with oil before shaping, I've found this will protect the grain side enough to make nice clean bends, as long as I'm careful when applying water. Skiving or sanding the area might be a good idea as well.

I used to have wooden blinds and the slats were surprisingly tough and flexible, probably birch or something similar.

[480volt]

Rattan cane can is quite strong, I believe SCA weapons are designed around it and it can be steam bent.I have used fiberglass rod for LARP weapon construction, very strong but you you cannot permanently bend it.