You can use any plastic shoe-boxed sized container i.e. Sterilite, etc. You'll know when they start to outgrow the containers - there is no exact set time or size. But for sure, they can easily outgrow a shoebox in under a year, so you'll need another rack soon. Keep one size rack for babies, one for yearlings up to subadults, and one for adults.
You can design it any way you please. I like tall and narrow just for space reasons in my room, but functionally any way can work for you.
I purchased a Nature's Spirit shoebox rack recently, and I have to say, for $160 shipped, it doesn't get any easier. You get 10 hatchling boxes perfect for BPs. It comes with back heat. Unfortunately in my cooler room, I had to add more heating capability to the rack, but no big deal - it works great! I think for that price you can't beat what you get. Same goes for Boaphile hatchling rack.
Another point - when keeping exotics like BPs or boas, it helps tremendously to heat the whole room to like 75-80, because then a small strip of Flexwatt can be the basking spot, or else the placement of Flexwatt on the back of the rack will be enough. If you use a rack with back heat in a cold room (say 65 deg), the boxes will not likely be warm enough for a BP. Cornsnake maybe, but BP no way.