Traditional sailor's ditty bags are tubular rather than square and always sewn from canvas sail cloth. However, they were hand sewn and not something that your average sailor these days carries.
I have made a few of these using traditional methods and they can be a bit tedious at times. I think the lack of curves in some bags might be due to the use of sewing machines and the need for relatively straight forward designs. Not any people hands sew canvas any more. Don't forget, even he so called 'hand made' bags that are sold in large numbers are made in an assembly line type environment to keep thecost reasonable.
As a side note of interest, when wind power ruled the waves the canvas ditty bag acted as a sort of CV for sailors. All the basic stitches to make a repair sails were used in its construction so a potential employer could see your skill at a glance. The knot work in the handles also demonstrated your skill.
My uncle has my great grandfather's to this day, although most of the canvass has now rotted.