Making a gorytos, part I

Over the coming months I'm planning to make an all-new gorytos for Marathon 2015.  While the gorytos was used for many centuries and came in several shapes, I only know of one that is attested in Persian art.
Relief on the Persepolis apadana

One of essentially the same shape is attested in some Greek pottery.  As near as I understand, these were made of soft leather with a wood spine sewn into the top to prevent sagging
Archer and hoplite kylix
Scythian on an amphora
Another Scythian

The upper edge is seamed; the main body may be a single piece folded up from the bottom or a front and back half.  Note in the second image that the bottom edge (the side facing down when the bow staff is worn horizontally) appears somewhat shaped to the bow staff.  This may be the result of leather being stretched, or it may be that the leather is seamed there - it's my understanding that fat-cured leathers don't mold well like veg-tan, and failing to stretch the leather sufficiently would leave the bow difficult to remove.  Many gorytoi seen in Greek art are shaped in this manner

A more extreme example that clearly illustrates the principle:
The bow- finding of olon kurin gol- some additional experiences and thoughts
This one is made with a wide, external wood spine.

Based on these, I've come up with some possibly reconstruction methods.

The choice between one- and two-piece construction and the exact widths required informs the choice of hides.