Here are my thoughts on the subject for what its worth...
If your going to do an exo, you might as well do it right. That means a complete exo. You DO NOT have a roll cage built in. It is roll over protection that our government requires. Again NOT a roll cage. The purpose of an exo is to protect the exterior. If you roll it, the roll over protection might keep you from crushing you brains but that is about it.
As far as design. The only way it wil look good is if it tightly follows the lines of thr vehicle. Before jumping into it, take a look at some of the 4x sites and see what looks good. You will quickly see which ones are half ***** slapped together and those that are well thought out.
Also use the right steel tubing. I am sure your friend will steer you right in that catagory.
Good luck.