apparantly it is impossible to make a laser fence but is it possible to make a prism support beam that links posts and any unit that goes into the beam will become damaged
The idea with the prism suport might work, if you can get the thing to continualy fire when there is power, then not fire when there isnt. But the thing is it wasnt meant as a weapon so it sont damage anything and its also way above any of the tanks hight. So try it and see but i think it wont work.