I too am cold in the winter and use something I heat up in the microwave. It is pillow like and filled with feed corn (the hard shelled kind like you find to feed animals). It keeps the heat a long time and is very soothing. I've also seen them filled with rice.

You might consider the battings that have come out for microwaving: Pellon Wrap-N-Zap Cotton Batting or Warm-n-Natural's version, maybe even Insul-shine (https://warmcompany.com/products/warm-home/insul-shine), even though you can't put this one in the microwave.

My "foot-warmer" is a piece of fleece sewed in half about the size of a half pillowcase. At the bottom of this pillowcase is a sewn line about two inches from the bottom. In that tube is about a cup of beans. This is put into the microwave each evening before I go to bed. I tuck my feet into the pillowcase, and I go to sleep so much faster if my feet are warm.

I also have an arthritic neck, so I have another tube, about 3 X 12, this one filled with rice. They go into the microwave together for 99 seconds. That's enough to put me to sleep and make my neck feel better.

Beans have a different smell when heated, but after using that bean filled pillowcase for more than three years, that smell is gone.

I think the wool would work much better than a synthetic batting.

I too, haven't seen a hot water bottle since I was a little girl. But I did heat wet wash cloths in freezer bags in the microwave ovens for particularly rough times of the month . . .a hot water bottle would definitely have come in handy ��

suggestion: make her flannel rice bags! She can heat them in the microwave for 1-2 minutes and they last for a long time. They also are wonderful for her feet and last almost all night.
I have a tube shaped one for my neck, and rectangular shape for my back and feet. They are wonderful.

Sounds a great idea, but she doesn't have a microwave.

For me, the cover on a HWB isn't to keep the content hot, but to keep the hot contents from burning my skin. Whatever you use, the contents of the HWB will cool. I used regular cotton batting with a polar fleece cover, nice and cuddly, for mine.

I make rice bags, too, but for around the neck I use cotton socks. For larger ones, I use terrycloth wash cloths and dishtowels. I sew tubes, then divide the rice into equal amounts. Pour some in, then sew that section shut; pour and sew, wash, rinse, repeat. I made one for my back using two dishtowels, so first sewed channels then filled and sewed across to make small rectangular sections. Using this method, the rice bag stays flat instead of it all shifting to one end.

When I get really cold, I use my heated throw. It is a lap size electric 'blanket' and use it to snuggle up and watch a good movie. Just plug it in and I believe there are about 5 different temperatures you can set. It's not too big, but not to small.