You could easily go down in tank size then. Really a 40 is huge for most BPs but you could use it when the snake is fully mature (or save it for another creature that needs a larger tank). Either a smaller tank or even better an Iris/Rubbermaid/Sterlite tub would be a much easier option for you. You could find a lovely Iris of an appropriate size at OfficeMax for under $20.00 (add in the cost of securing the lid with big clips or straps or whatever). Iris in particular makes a really nice brand of tub, they are glass clear and have smooth insides so no snake poop gets stuck in grooves.

Here's a site for a great guy Eric Erb who features lovely Iris tubs. Alternately you could go with Rubbermaid or Sterlite which can be found just about anywhere. If you are looking for a smaller glass tank phone around to aquarium shops, they almost always have spare older tanks that may not be water tight but do beautifully for snakes.

http://www.reptiletubs.com/