Large styrofoam cup with hole poked in it. Large plastic cup with hole poked in it. Large plastic container with hole poked in it. Water jug with hole poked in it. You can also use all above methods, but instead of just poking a hole, you can put a small-diameter plastic tube in the bottom and seal around it with silicone. Tubing allows you to control the drips so it doesn't drip constantly into the soil of your plants.