[zubspace]

Maybe just one or two hot water bottles? [1] We use them in Winter to warm our feet in bed. But they are very versatile and can also be used to alleviate any kind of tension.

I also use one while I code: I put it inside of a small (baby-)sleeping bag wrapped around my feet. Sounds weird, but it's so comfortable!

[1] https://www.google.com/search?q=hot+water+bottle&tbm=isch

[ArmandGrillet]

Went for https://www.yuyubottle.eu/ after a bit of googling due to the cotton fabric + the strap, will test it this winter.

[sixstringtheory]

We use this trick on winter backcountry camping trips to good effect. After dinner, melt snow to a boil and fill up all our water bottles and bring them in the sleeping bag. Every little bit helps!

[gima]

Another trick is to put lots of dry rice grains into a sock, tie the sock and warm it up in a microwave. Just be careful, as the rice heats up Really fast in the microwave.