I've had good luck with the cheap MotoGear tailbag ($30 USD at Walmart).
The one in the picture below is 5 or 5.5 years old and I've used it for over 20K miles on two bikes. It is the "gray thing" under the black camelbak pack. UV in our desert environment tends to bleach everything. You do need to use the (included; lives in a zipped foldover at the front of the bag) yellow rain cover in hard rain.