IF a 10th century Byzantine carried a leather bag, it is likely it would have been similar to the basic satchel of the soldier, pilgrim, or book carrying scholar. So, simple rectangular bag roughly 18 by 12 inches, with a flap that came over the top and fastened down. Inside might be smaller leather pouches holding smaller items.
A whole morning spent thinking of search terms for online and the most useful searches turned up wikipedia articles on the luggage of the late roman soldier, specifically the Loculus and the sarcina, and a bag from a pdf: 1st Century Leather Roman Pouch
I read a bit of Leo the Deacon’s history where the satchel is mentioned. I looked at any number of web sites with images. The poor, the pilgrim, the soldier all are pictured carrying satchels.
Silk reliquary bags were shaped the same as leather money bags. Here is another nice collection of images and links. Bags for books and pilgrim’s bags seem to have been decorated, but the Roman Soldier’s bag is wrapped with reinforcing straps and not decorated in the images.
Wealthy persons are not pictured carrying bags. My persona does not have any servants in the current middle ages so she needs a bag and some pouches. The smaller roman pouches found seem to have been carried inside the loculus so I shall imitate that practice.