Clearly in nature, they burrow. Not always practical with breeding, I have used potting soil before, as well as dry moss in containers.
I've had the best luck using a small plastic bin, like a small animal cage with a slotted lid filled with moss that is dry and sterile.
I've used soils a couple of times, some larva went right down, others would wander the top for days, next time I have burrowing
larva, I'll be using the moss and saving the money I spent on organic potting soil.
I keep track of dates, and the amount of larva in each bin, a post-it-note works fine to write down details. I've put as many as 10 larva
in a small to meduim cage, perhaps 2 liters at most, it depends on when the larva are put in, you don't want larva disturbing each other.
If it's shredded newspaper or other medium you prefer, very well, I have a supply of fluffy moss, and it's free.