KatherineK16, thank you for your advice! I'm already using hot water bottles and I find them really efficient. I use them mostly to relieve the pain on my sternum when I start to have some difficulties breathing or on my belly when I get cramps/pain at night. I really like the idea of adding a knit cover, thank you! :-)

FayeK, I love your idea! Unfortunately, I don't have a microwave, but I think it is such a nice idea :-)