Keep a couple of things in mind:
1) it is going to be hard to add a sheath to the back of an already-made bag. I have stitched inside a bag, and it isn't easy.
2) Unless the sheath is rugged (and therefore hard to stitch on), you run the risk of punching the knife tip into the inside of the bag. It maybe period correct, but I cast a suspect eye at some of those sheaths on the back of bags. I think we have learned a lot in the last 200+ years.
3) A sheath can be attached to a bag strap, and that might be easier to access.
4) In my opinion, a sheath on the waist belt is an easier-to-use choice. I think this was the majority of users' choice back in the day.
5) If you sew a sheath on the back, you are locked into one knife. If it breaks or you lose it, any other knife is a compromise.
I hope this gives you good food for thought. God Bless, Marc