I considered a hot water bottle, but I most frequently use these things when I have a headache, and the smell of plastic makes me sick when my head hurts.
I also considered doing grains in the oven, but between how long it would take to get them hot enough, and the energy expense, I didn't feel that was practical. Over the winter, I could have kept a shallow pan of grains on top of the woodstove for this use, but now we're only using the woodstove at night. I guess the oven method is the only way, though. Hm... I wonder if they'd warm up more quickly if I put a pan of boiling water in the oven too? The steam would help heat them more quickly than dry air.
Thank you for the help!