I forgot who it was, but I recently watched a video of a HEMA practitioner on youtube who talked about the difficulties of keeping a weapon of war (polearm, twohanded axe, twohanded sword) in a scabbard on your back. And it basically came down to that if you want to carry a big weapon around with your hands free, you need to use a modified gun sling. It’s loose and easily taken off your back, which is essential because you need to completely take the scabbard off your back before you can get the weapon out of the scabbard.
Of course, if it’s loose, that means zombies can pull at it.
Then again, you’re using a bigass twohanded weapon. If the zombies get close enough to grab your straps, that’s your mistake.
Exemplary image.