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!