I don't know what I would do without hot water bottles in cosy knitted covers - such a comfort when feeling seedy. I just bought two new bottles - which meant two new covers to knit. I have made many covers down the years but find that plain stocking stitch is nicest to cuddle up with. I don't follow a specific pattern but just knit to fit, starting at the top of the button band, working down to the bottom and back up until I can cast on for the buttonhole band and then carry on in the round. At the neck I cast off at the beginning of one side and for the other side I fold and do a three-needle cast off - so I work flat from that point but only have one side to sew up. It sounds complicated but is really easy to do and makes a well fitting cover as you can try it on to get exactly the right length to the shoulder.