You can keep the hatchling in a 10 gallon tank which will last it some time, then as it outgrows it you can switch it over to a 20 gallon. I would say 20 gallon is the minimum you want for an adult, but 30-40 gallon is more appropriate.

As for cage items, I like using those exo-terra caves for hides, they work awesome for any snake I have. I use it for one of my boas, one of the BPs and for one of my Bloods. Anything works really, but try to get more secure hides with only one entrance. It doesn't have to be tight-fitting, but not incredibly large either. Two identical hides for both ends of the enclosure (one on warm end, one on cool end).

For bedding, I like using aspen bedding for colubrids. But, it's totally up to you. You can also use cypress mulch which is OK to say the least, newspaper, papertowels, sani-chips, or coco fiber (comes in a block, use warm water to break it apart).

Other cage items is your choice, you can put fake plants or logs around the enclosure to make it look nicer and more presentable (which can go along with the more natural looking bedding such as cypress mulch or coco fiber).

I've never necessarily had a problem with not covering 3 sides of the enclosure, every corn I've had and every corn I've cared for has normally eaten. Every corn I've personally had would have no problem eating and not much stress even though I never covered 3 sides of the enclosure. I believe Ball pythons get more stressed out leading to covering 3 sides of the enclosure to make them feel more secure.

As far as humidity goes, I find about 50% to be pretty reasonable. Don't let it get too low or the snake won't have proper sheds. I've also noticed the scales will flake off with low humidity levels (not the actual scale, but a clear piece that looks like shedding).

Good luck with whatever you get, post pics when you get it for sure!