Agreed, the 4' by 2' cages are great for adults. I'm planning on purchasing these for my females... but I will keep the males in underbed storage bins all their lives (unless they get to be monsters!).

Also, www.reptiletubs.com has a selection of tubs that come in large sizes... the VE series could house a boa---even a female---into it's adult life.

I think you can also buy these at the Container Store...