Eh.. a 20-long sounds like too-big of a tank for a hatchling. I have my adult corn in that size tank and it is fine, but he is 4'.
I would get a nice tub from walmart or another store like that. For a hatchling, one of those slightly-larger-than-shoebox tubs would be fine. You could use newspaper substrate and should have identical hides in it, one for the hot side, one for the cool. Then go on and read the caresheet on this site for all of the temperature criteria, since it is on there it'd be useless for me to retype all of that :)
You could either sell the 20l, or keep it for when your snake is a bit larger.