Woah woah woah. I feed conservatively, even a bit more than conservatively and I have never seen a one year old boa female outgrow a 20 gallon. On rare occasions have I ever seen an adult female that needed anything more than a 4x2x2. Absolutely do not put a baby boa in an adult sized enclosure, you will only stress it out. Appropriately sized enclosures help the animal feel secure. 6' AP enclosures are more appropriate for burms and retics. Boas grow MUCH slower than these other giant snakes. If you are going to stick with glass aquariums that's fine. Do the 20 gallon for a couple years then upgrade to the 40. You can foil the lid, or put a piece of melamine or plexiglass over the top to keep humidity up and limit heat loss. Everyone loves to hate on aquariums, but they can be adequate until you are ready for it's adult cage.

There's also no need to wait for an 8 month old baby. I've been breeding boas for years and the general consensus that I and all other breeders I know is one maybe two sheds, a couple meals and you're good to go. Never had any complications with a baby that has atleast eaten a few times.

People get nuts (no offense) on these forums with caging requirements and growth rates. You will find that a baby boa grows at a glacial speed compared to retics and burms. You have plenty of time to figure out an adult cage. Just make sure the basic requirements are met now.