Some of the best cage building info I have found and read is http://www.arbreptiles.com/cages/tips.shtml
Clay is a great guy and builds lots of caging
http://www.arbreptiles.com/cages/cages.shtml
And look there, there are a few of those designs that would be perfect for boas.
If you decide mot to build one, I would go with boaphiles. Pretty reasonable in price and GREAT cages