S. aloeus, or S. anteus?
I don't have any experience with S. aloeus, but I do with S. anteus.
The substrate mixture I use for the S. anteus larvae is 90 percent fine sand and 10 percent soil (the dark, "composty" looking top soil). I fill up my rearing tank with about 6 or 7 inches of that mix.
Then, I bury dog food and old, brown leaves in the center of the tank. Every few days, I dig up the old dog food and replace it with new dog food.
As for S. aloeus, I'm not sure. Maybe Orin can help out on that.
Cheers