My hatchling corns were always set up in a 10 gallon tank with securely clipped lids...they are escape artists. I mostly used toilet or paper towels tubes as hides, about the only thing small enough. I also included some branches for climbing. The ambient temps in my home were fine when I had hatchling corns, room was mid-to-upper 70’s year round. I didn’t do anything to boost humidity, but their water bowl was large enough to soak in if they chose to do so...they never did.
In my current home the winter temperatures are too cool to not provide a heat source, and my corn is a 16 year old adult. He’s set up in a Reptile Basics 3’x2’x12” cage with an Ultratherm heat mat on a Herpstat thermostat to provide a basking area of 85-87F. Ambient temps in my snake room are 74-77F. He mostly stays on the cool side of the cage except for right after a meal, and then rarely right over the heated area...mostly to the edges which read around 80F.
Ultratherm makes a small heat mat that is 6”x11” that is perfect for a 10 gallon tank or small tub (see link below). I’m using this size for Selene’s quarantine tub on a Jumpstart thermostat (link below) in an upstairs room with ambient temps of 77-79F. I think this would work pretty well for a hatchling corn.
Edit: Forgot to answer substrate...I used paper towels for hatchlings and juveniles...they are pooping machines. Easy to see waste and easy to clean.
http://www.reptilebasics.com/ultrath...ater-uth-6-x11
https://www.amazon.com/Hydrofarm-Ger...art+thermostat