Hmm, this is tricky. The problem I forsee is that the collar will inevitably need to be larger than the quiver. This is due to the leathwr thickness. The collar will end up being larger by the thickness of itself and the quiver leather thickness. You might be ok using a thin leather that would stretch better. Since it is decorative, I'd recommend 2/3 oz. However, if you are to use thicker leather you'll have to figure the dimensions of the collar.
If it were me, I would have my 3.5 inch dowel, wrap my quiver leather around it, then wrap my collar leather around that. This gives you the proper length for the collar.
For gluing, I believe you would need to glue on a curve so the quiver and collar match up (again because the collar will be wider) then stitch. It won't be flat exactly but it won't be a complete cylinder either.
Dwights method is far simpler, but I do feel it depends on what type and thickness leather your collar will be made from. And honestly. I would try dwights method first (on scrap) due to its ease of execution.