And... I guess if you CAN'T sew...If you can sew even just a little bit, there are several how-to directions and videos on YouTube that show how to make a microwave heat bag (some use wheat, some use rice or corn).
Making-A-Microwave-Heat-Bag
homemade-heat-packs (Pinterest has a lot of links to how-to's)

Just kidding!

Seriously though, always (over) thinking...

If you're like me and maybe too disorganized to find a needle and tread
Maybe tie some into an old mismatched sock that's hanging out in the drawer. 
ETA: Beautiful work there.

Having tried many I’ve found this to be the best one. At least for me it is, everyone is different though.
The reviews were a bit mixed but like York says, different things work for different people.