I would also suggest tubs for neonates to sub-adults. Start off with a shoe box and two hides one cool side, one hot (don't worry about placement too much, 2 of mine move their hides where they want them). Move up to larger size tubs, and hides as your snake grows. This will be very inexpensive, and you'll have more time to shop around for the right enclosure that suits your snake and your needs.
Tubs:
Iris - My favorite because they seem to keep the same design year after year, whereas the other two change: Container Store, office supply stores
Rubbermaid: Target
Sterilite: Target
Hides:
Reptile Basics
I would say a 48x24x(12|18|24) would work for a large adult female at 1400+ g. Put a divider in, and it will hold two sub-adults, one on either side. Don't forget 2 hides each.
Height: your BP doesn't need a lot of height, and will do fine in a tub that is only 6" tall. You can use an enclosure that is 18" tall (rather than 12") so that it is easier to view and clean. If you use a 24" enclosure, you have more versatility later on; if your collection grows and you move the BPs to a rack, you can convert the enclosures to use with arboreal snakes such as GTPs or carpets. The taller you go (24" MAX), the more important it will be to add items to the enclosure to make your BP feel more secure. I wouldn't add items that would allow the BP to climb higher than a few inches.