I remember building a tiny turret capable of fitting in the 7x7 default, making a 10x10x7 room at a major corner of my ship, and mounting the little turret to the ceiling. It only had like 3 or 4 AMCs, and I'd just set the range as low as possible so it wouldn't try to target things outside the ship. This also let me put a lot of points into RoF and damage.
I tested with some friends, and it wrecked intruders, especially if the turret was backed up by an assisting crew member. The crew member back up was needed because turrets only target one thing at a time, so one person could jump around avoiding the turret while it focused on them, while the other boarders destroyed the turret with focused fire.