I use milk jugs for watering my potato bags. I punch three holes in the bottom of each and then fill 1/3 with as big of rocks as I can get through the top. The rocks keep the jug from blowing away
I made the bags out of some leftover landscape fabric and they are 18" in diameter and about 16" tall. I roll the bag down so it is about 8" inches tall before I plant in it. To plant, I put 4" of soil in the bag, then 3 or 4 starts in the bag - halfway to the edge, top with 2 inches of soil, and then place the milk jug in the center of the bag. It sits between all the starts.
I have the potato bags surrounded by wire mesh and that is covered with greenhouse plastic to protect from cold night temps, drying winds and gophers.
To water, I just take off the top and fill the jug. The holes are small, the diameter of a bamboo skewer, so the water seeps out slowly and saturates the soil without running out. The lid seems to help keep the water from running out too fast and also keeps the insects out of the jug.
As the plants grow larger, I just take out the milkjug, unroll the bag a couple of inches, add more soil and put the jug back in.
On really hot windy days, I may need to fill the jugs twice - once in the morning and again in the evening.