Yep you are on the right track there about splitting the head up the back. About the only thing you can do after the fact.
It looks like maybe you used felt for the straps and it well stretch. You can try to stuff some foam inside the felt pocket to tighten it up if you don't want to dig into the back of the head. I would say in the future to use something that does not stretch. You know the straps you find on back packs? It makes good straps and won't stretch. Google Search:
Nylon Belting