4x2x15 would be fine and so would 3x2x2 OR 2x2x3. All of those are more than sufficient. ATBs will stretch out longways just as much as... upwards, so as long as there are perches, the overall height isn't going to be a deal breaker or anything.
A tub is generally better than a tank since humidity is much easier to hold. I use green garden fencing in tubs (in QT or for babies) and then perches in cages. Yes a UTH is fine for now.
ATBs generally spend quite a bit of time on the ground when sleeping during the day and will usually happily make use of a hide if provided.

Here's some pics of mine and my set-ups. Good luck and enjoy your little demon shoestring!
https://ball-pythons.net/forums/show...2-Wall-of-ATBs