Putting him in a 20l won't hurt him at all of you think he 'needs' more space. He doesn't at all but eventually you will bump him up to one anyway unless you go a diff enclosure route. Since you work at Petsmart, aren't they still doing the $1 per gallon sale? I know here that ends in the 27th. If you do that though, add some plants etc to clutter it up. They like to hide a lot and will feel more secure.

The hydrofarm thermostat is good for a starter; easy to set up and inexpensive. If money is tight, it is the way to go. If money is not such an issue, a herpstat from Spyder robotics is the way to go. Given the margin for error on the hydrofarm <+\-4> if you go that way, start saving your pennies and try to upgrade to the herpstat later.

I too live in the desert, thankfully have had thunderstorms come through to naturally bump my humidity but it is a pain. The foil/tape trick is a godsend. The 20l will also allow you the space for a larger water bowl which will help as well. Until then, a spray bottle and/or a damp towel laid over 75% of the top will help you out.

Once set up, leave him alone for a while to get settled in before trying to feed. I too suggest live for your little guy, switch to f/t after he grows up a bit.