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carradice is a good style to copy if you rig up a bag
there's a thin internal dowel and then holes in the bag which the flat leather straps go through, and loop around the dowel.
Good way to add some stiffness/shape to a bag and it means you don't necessarily need to sew anything to add the straps.
wooden dowels of all thicknesses are cheap/easy to buy although ime old dowels can be quite a lot stronger than the cheap hardware store pine ones you get now so worth looking for old ones.