I have the same problem - I would love to have a go at making some bark containers (I've practiced with cereal packets - yes, I am that desperate!)
I have seen US sites that sell fairly large sheets but its prohibitve in postage to get.
Any ideas?
I think willow bark will become a bit too stiff to work well. I've made the so called "berry baskets" from willow, and after a while they become stiff and brittle.
The basic pattern for this kind of birch bark containers I was taught is that the end of the tab is "arrow-shaped", and that the inner layer then covers the "joinery". The bottom fits inside, and small wooden pegs lock it in place