Hot water bottle covers where something I tried a couple of years ago, when going through my daughters baby jumpers. some where a little worse for wear, or too loved, to leave our household. I found some knitted ones, with stretchy enough necks for me to slide the water bottle through it, then I traced a hot water bottle onto the (inside out) jumper, and just sewed around that line. Bingo!
zip up cardies, and larger than baby clothes too.