Here are plans: http://www.blueappleherps.com/page13/page5/page5.html

This is a viv I designed. Total cost is ~$100 not including heat and it will hold two snakes. I designed it with cornsnakes in mind, but it can easily be used for ball pythons. Instead of using 12" wide melamine shelving - use 16" wide shelving all around (everything else would be the same though). Also, if you're worried about humidity, you can just use hardboard instead of pegboard and then drill ventilation holes. I also designed it to be built by someone with no experience and only using a drill and tape measure.

If you have any questions, don't hesitate to ask.