One of our hot water bottles developed a leak a while back, which obviously means it can't be used any more. I was reluctant to throw it away though and looked for ideas of how to repurpose old hot water bottles.
I discovered this idea somewhere on line (I didn't make a note of where, which was an oversight.) and thought it sounded like a good idea!
It's really simple, though it takes quite a lot of time to do, you need to cut up a lot of fabric! This can make your thumb sore, so you may want to just cut up a bit of fabric at a time and spread the project out over several days (which is what I've done!). I've got lots of scraps of fabric, left over from making chopstick bags, gift bags and other items.
Basically you fill the hot water bottle with scrap fabric, that you've cut into small pieces. Make sure the fabric gets into the edges of the hot water bottle. Stop adding fabric when the hot water bottle feels comfortable to kneel on.
This is great to use in the garden or for any indoor tasks that are easier to do kneeling down. It really does protect your knees!