Well, it'd be kinda pointless
opponents very rarely ever go to the back, so, if you want a back weapon, I would suggest a saw or a spinner (ala Sergeant Bash or Matilda). a Hammer would require precise timing and have a little bit of delay where as a spinner would damage the opponent as soon as it touches the weapon.
all in all, backwards weapons aren't all that effective and I'd discourage you from building one, but if you want to build one anyway, a spinner would be the way to go.
However, if you
want to build a hammer or a piston...
Use two smart zones.
this might work.
