Additional info: when my quiver arrived from HHA, it was oval at the top as well as the bottom. Evie even shipped it in a roll of soft corrugated cardboard, so the shape was evident even before I unwrapped it:
The large oval opening indeed caused the arrows to rattle around quite a bit.
Remembering things which I had read, and photos of Howard HIll and John Schulz, I realized that their quivers where quite flat from the middle to the top. Then I read Greg's TBM article. As he mentions in his article, this flatness allows the quiver to pinch the arrows in place when the quiver is worn on the back.
To achieve this I flattened the upper half of my quiver by rolling the top of the quiver down (rolling towards the back side of the quiver, i.e., away from the string pouch), and then setting some weight on it overnight. The top half of the quiver and the stiff upper cuff collapsed somewhat after this.
Now when I wear it, the arrows stay fairly quiet inside. Since the fletchings are close to my right ear, I can hear some soft "rustling" coming from them when I walk. But it's not very noticeable. What helps with this, as Greg also mentions in his article, is that the quiver is designed to be worn at a angle. He says that
"the arrows should be lying in the quiver at an angle, not straight up and down".
To achieve the proper position of the arrows, you need to adjust the quiver strap, again as Greg explains in the article. He states
"The quiver should be adjusted so the strap is fairly snug, but able to be rotated freely when needed. The strap should go from your neck on the opening side to above the level of your nipple and under the arm of your bow hand. This places the arrows around the level of the top of your shoulder or just below".
After making these adjustments I can now bend way over without the arrows falling out.
The oil treatment should also eventually aid in making the stiff leather more pliable, allowing it to better collapse on itself.
In retrospect I think that if I had ordered the "soft leather version" from HHA, I would have been better off. But the one I have should hopefully improve with use.
Best wishes,
Frank