My brother and I argue over how the archaic dudes mounted their grooved axheads. He thinks they did it how the later guys did, wood handle up to the head and rawhide holding it on. I think they carved the handle out to fit the groove, then rawhide. Either way, rawhide is the key, because applied wet, it gets real tight and hard when it dries. For a wallhanger, you could use leather or artificial sinew or maybe some hemp cordage.
Nice job making that, the groove is a little too deep, but once it is mounted it will look good. The best handles are found exactly where the natives got them---the forest.