After placing the plastic around the inside of the garden bed, I placed a single layer of cardboard on the bottom of the bed. A great place to get cardboard are your local cigarette stores. The boxes are small enough to put in the bed and will not be too bulky.
DIG Irrigation Products drip and micro sprinkler system (not a sponsor, just love their stuff.) DIG has created an awesome FREE Drip Irrigation Installation Guide which guided us through what to buy and how to put it together