For a simple heat source,
a Wheatbag is the best idea.
I have two and cannot live without them.
They heat up within minutes in the microwave and give warmth for a long time.
They do not get cold like hot water bottles but keep the warmth of the body.
I love to heat up the bed with them in Winter.
And here is how I made them:
Use some leftover material you like and sew up a small square
like a pillowcase.
( 20cm by 20cm or 8 inches by 8 inches).
Leave it open on one side.
Buy some Barley from the store
and fill the bag with about 1 kg Barley.
Once the Barley is in
sew the remaining side together,
and your bag is ready.
I also made some covers for mine, just like a pillowcase,
so the bag itself stays clean.and
I can wash it from time to time.
When you need some warmth,
put the Wheatbag into the Microwave for 2 or 3 minutes
and enjoy the warmth coming from it
on your legs, on your feet, everywhere.
It can be used for aches and pains too, i.e. like stomach pains, etc
They are very versatile
and can be used cold too.
I have made some small ones (10 cm by 10 cm)
as they fit nicely into a bag into the freezer as ice packs for any injuries.
You can if you like put a bit of lavender inside too and it keeps smelling lovely too. I do like the smell of heated barley though.
Enjoy the warmth!