I have made 4 roll cages for my race cars, in my opinion roll cages perform two main functions once installed into a car. Firstly and obviously they protect the ocupant from harm in the even of an accident, secondly and more importantly it stiffens the chassis to maintain geometries of the wheels on the road.
Roll cages are an art to build correctly, not only to design but to actually install. A badly built and installed cage could be a waste of money and be dangerous. if its not designed and built right it may fail when it is needed most, if it does it may be more deadly than the accident in the first place. many a home made cage has collapsed or sheared during an impact, leaving unrestrained steel bars to fold onto, or pierce through passengers and drivers...trust me it happens.
The geometries of triangulation, while not difficult to achieve are all important to success.
Roll cage material is also important. Cold drawn seamless (CDS) bar is common, whatever you call it in the UK, moly and alloy tubes all vary in thickness, but of course the thickness is important. There are rules governing the cage design in most forms of motorsport, drags and trackdays may not though.
Bends need to be mandrel and cannot deform the tube, the welding needs to be tight, neat and seamless and to prevent failure should be quality welded.
Are you going for a weld in cage or a bolt in. if its weld in make sure you install the main parts or raised feet, last, to allow you fully weld around the top tubes before jacking it up into position and installing the feet, otherwise the cage will be hard up against the roof sheet and inaccessible.

Good luck, hope this helps

Justin P