Not much is going to get me to post publically around here, but this issue is one that is critical. The design and creation of a safe cage is just not something that anyone with a MIG welder and a harbor frieght tubing notcher should be attempting.
1. There is alot that goes into the proper design of a good roll cage.
2. Heat affected zones must be dealt with appropriately or you'll have a cage that is simply not going to do it's job when it counts.
3. Even the tubing must be picked carefully DOM tubing is considered a must have for uniform wall thickness.
4. Bending hoops is NOT easy, They must be bent carefully without creasing the tubing.
I might, build my own cage, but even then I would run my design by a real Mechanical Engineer with FEA software to insure that I was not doing something stupid as well as every track person I could find.
If you MUST build your own cage, then remember that the triangle is your best friend. It is the strongest structure that you can choose. DO NOT USE UNSUPPORTED SQUARE STRUCTURES. X Brace everything you can within reason if it looks like a square or rectangle.
Your welding must be top notch. YOU SHOULD TIG WELD ANY ROLL CAGE IF POSSIBLE. MIG is acceptable but you better be damn good. Weld Pentration and Heat Affected Zones are critical. Roll hoops should ALWAYS be one continuous loop of tubing consisting of less than 180 degrees of total bends.
When in doubt examine rule books regarding cage specifications. The more rule books the better, find rules for cars that exceed the performance of your car and weigh the same. That may give you a good grounding on why it would be prudent to choose a professional to build your cage for you.
Mark