I did heat sand up this morning in my little office microwave, I threw out my back and was looking into making a heat source to put on it, not have much out the office that I thought would work, I tried sand, ran some magnets through it to make sure there was no metal of any kind in it, it was all good, so this is what I did
1 - cut the strap off a junk back pack, then cleaned out the inside
2 - Taking sand I bought from Home Depot I filled up the strap with sand and zip tied it shout, leaving room inside for it tot move around
3 - place in the microwave and heated it for 30 second burst until I was happy with the heat it was producing
Seems to be working fine at the moment
Sad to say I had the "hey what if" in me doing this before ANY research was done, but it seemed to work fine so I guess I got lucky, LOL :)