Seeing as how you're somewhere in Mexico, I'm sure there's more humidity along the coasts than towards the northern interior, I would stay away from sand as a substrate.
Sure, some are marketed towards reptiles, but corns should never be one of them. Sand has a drying effect and the tiny grains can get under scales and cause sores, and the possibility of death due to ingestion is also a major drawback. I wouldn't even use sand for desert species, just too much risk associated with it, in my honest opinion.
Most folks around here use aspen shavings, or fir bark shavings. Newspaper or paper towel are totally acceptible as well.
Stay clear of pine and most definitely stay away from cedar. Both of these have oils (that make them smell really good to us) that constant exposure can damage the respiratory tracts of small animals.
A humid hide is one of the better ways to increase humidity when the snakes need it without over-doing it. Corns are from a temperate region of the US and do not need constant high levels of humidity. Too much humidity can be just as bad as not enough. I've never had the need to mist any of my snakes' vivs. Most of the ambient humidity is provided by limited airflow and placing the water bowl on the warm side of the viv. The warmer water increases evaporation into the air.
So your sandwich container with moist sphagnum moss is an ideal solution to counter potentially low levels of ambient humidity.
That said, I've never had a bad shed due to low humidity levels. The levels in my home are around 20-30% and all of my corns shed just fine. The only bad sheds I've had are the result of an otherwise weakened snake.
Grats on your new snake and welcome to the forums, too! =D