Here is how I round out the back of the handle. I use thick leather, cut a 4" rectangle and shape it...
...then I soak the leather in warm water to make it more flexible, add TB glue to the leather and back of the bow handle and put them together. Next I wrap with rubber bands until the glue sets(usually a few hours)...
...and here it is once the bands are off. After this I shape the leather so it transitions smoothly to the back of the handle.
Once it is all cleaned up I like to use hemp cordage for a handle wrap like this. You can see the filled out bulbous handle in this pics. It really fits the hand nicely...