I’m looking to make my own “popcorn salt” by pounding the heck out of regular salt or putting it through a food processor. I’ve done this once before and it ended up caking annoyingly.
Is there anything I might have around the house that I can add to prevent this?
rice, beans or crackers in the shaker…I use the crackers for sugar shaker, cause the rice will fall out the hole. 
Cornstarch used to be used as an anti-caking agent in powdered sugar. It’s not Calcium silicate, but it might do the job for you.
According to this site, about 3% cornstarch (presumably by mass) was the usual dosage.
I also vote for rice, it seems to be a moisture magnet
Definitely rice. If the holes in your shaker are too large you can also get a little disc of unglazed but fired clay.
Yeah, my first thought when I saw the OP’s thread was, “What, rice ain’t workin’ out for ya?”
Definitely uncooked rice.
I’ll try rice, then. The problem was mostly it sticking to the sides/bottom of the glass container.
I added a little bit of cornstarch but it didn’t seem to do much, if anything.
When I worked in a movie theatre for our popcorn salt we used…wait for it……wait for it…
Unpopped popcorn kernels (really no different than rice, but the beauty is in the symmetry)
A bit of sponge. Or the little package of dessicant you find in a bag of jerky. It’s the best remedy I’ve ever found (though I’ve never thought or heard of cornstarch till today). I’m sorry, but rice etc. has never worked for me. Maybe it’s just the humidity factor, possibly that rice can’t cope with the high humidity of where I live, or I don’t know what it just rattles around in the shaker and breaks into bits clogging the holes along with the caked salt.