I know you're looking to make a bag, but this might give you some inspiration: http://www.banjobrothers.com/products/minnehaha-bags/

I have one of their rack top pannier bags and like it quite a bit. It's nylon and not what you're looking for, but it's a good design. Their other products seem well designed and made too. The Minnehaha line is canvas with leather straps. It's probably the line that you would want to purchase or emulate.