I bought these, just got them and haven't tried them out. They hold 1.25 U.S. Gallons-about 4 liters, i think--don't know if you can get them in europe. They have a mount that attaches to a pannier and makes them easy to carry, also they make one to carry water (white).
Here is a link to the manufacturer;
And here is a retailer;
And here is a photo;
OK good luck