You'd probably do fine with a 27-32qt tub for a boa. Ditch the Reptibark, it's incredibly dusty and expensive. Newspaper, aspen, paper towels, even dried cypress mulch from your garden center(Home Depot has it for $2 a bag).

For a bigger tub, put about 6-8 holes on the long sides, and about 5-6 holes on the short sides. None in the lid. Water bowl all the way over on the cool side and make sure it has a small surface area(2-4" in diameter).

Too many holes will just let the heat out and will make the tub too drafty. Cool drafts can predispose the snake to respiratory infections.