My old hot water bottle cover fell apart and a knitted one my Mum made was saggy from the start (sorry Mum!).

Thought I’d have a go at this using miniscule lengths of yarn left over from my blanket (on edge of photo).
It really did use next to no yarn.

One thing to note is that I am a loose crocheter and I ended up using a 3mm hook to get the gauge (and even then I was very slightly too big). To compensate I did only one round of htr (US: hdc) as a border.
I also played with the neck and used a bigger hook halfway through to make it slightly looser where the bottle opening gets wider.
I finished with crab stitch (US: rev SC) to get a nice, firm edge.
Overall making such tiny grannies was a bit fiddly (esp the weaving in of ends) but the result is worth it, I think xx

Made a second one using scheepjes yarn leftover from my Moroccan Tile Wallhanging. This is pretty, but I’m finding that the yarn bobbles quite badly. This is not noticeable in the wall hanging because it is not handled.