I use rice to fill heat packs all the time. It is cheap, always on hand and no issues for anyone allergic to wheat.
I have heard of other seeds and things being used to fill them, just don't use corn! You will end up with a tasty snack!
Lol- and an explosion when seams can't hold the popcorn in!!
I may give a pair a try. My hands freeze
The way I was taught was size 4x4 and we filled them with pie weights!
They are little balls sold at craft stores or kitchen stores! I said the same thing what are they when I was being taught how to make microwave bags! I was told to make 1 with rice and 1 with pie weights and boy do the pie weights get really warm and stay warm for a long time!
Check out littlehouseliving.com she says 4x4 flannel and one comment says to put cotton in between flannel. Hope it is OK to post the site.
I think she said on her list of supplies cotton flannel or she means cotton fabric because to put in microwave everything including thread has to be cotton. She said hers are cut 4x4 so finished would be 3 1/2 in sq if using quarter inch seams. Thanks