So this may have come up before but I couldn't find it.
My suggestion is simply to make a check box that allows you to swap the weapon "attach to back" with your current weapon.
For instance you are carrying an axe killing zombies and you start getting overwhelmed; you have a gun attached to your back so you pull it out causing your axe to be equipped to back instead of the gun.
After you finish with the bulk of the zombies you swap back to the axe without having to open your inventory.