1: Make a small box (any texture, it will be invisible in the game) 8 units wide, 8 units long and 16 units high. The base of this box must be ONE unit above the top of the wall.
Does this mean a entity box or a box made by the block tool?
2: Tie this box to a func_tank. In the old Half-Life, this entity served as a controlable mounted gun. It serves the same purpose in Half-Life 2, but in HL2 the func_tank uses the prop_dynamic as the visible gun.