On my Survival Island map, (mentioned here) I converted a double dungeon into a Mob Farm. I first started by making the walls even and square. I then made the dungeon a bit wider. Once that was done, I dug a 2 block deep by 2 block wide trench down the middle. Then I added water. Since the mobs could get stuck behind and in-front of the spawner, I made a wall on each side that prevented this from happening. Once the mobs are forced to the end of the trench I can easily kill them.