Every SL-C ships with a 6-point cage as standard equipment. Alternatively, a more focused track or race cage may be ordered for installation when the car is being built.
This page discusses the standard cage.
The cage is built of 1.5" DOM steel, and consists of a hoop over and behind the drivers head, a hoop at the edge of the windshield, two bars connecting the two hoops, and a pair of backward-facing stays that tie the rest of the cage to the chassis in the rear.
The cage is usually preserved with a coating of paint, powdercoating or lately, a nickel plating.
Of course, many builders can't leave well enough alone, and so several of them have made modifications to the cage. If you do so, please employ the services of an engineer to be sure you don't compromise the safety of this key component. Don't get happy with a torch and a welder- know your limits and let the pros take over when safety is a factor.
Relocating Rear Stays
Some builders (and the factory) have relocated one of the rear stays to allow clearance for an engine intake that winds around and down before the rear wheelwell to mount an air filter, instead of just running a straight tube back. The stay on that side needs to be moved forward to allow clearance for the intake tube.
Another modification adds stiffness to the cage. While the aluminum chassis is incredibly stiff, and much stiffer than any similar-looking car, bolting in the cage actually adds even more stiffness. To further increase the overall chassis stiffness, the cage could be easily modified by welding an 1/8" plate across the length of the two tubes that tie the two hoops together. This simple mod makes the cage much stiffer, and so adds to the overall chassis stiffness.
Re-shaped Front Hoop
Another mod that one builder made was to cut off the front hoop, and re-make it to more closely follow the shape of the windshield. Making the hoop fit very tightly to the windshield or body is standard custom cage practice, so your local cage fab shop should find it relatively easy to do. The advantage to this is that the FG A-pillar covers can be sectioned to be much narrower, and thus improve the range of view from the driver seat.
Not really a cage mod per se, but a very popular add-on. These are often added to track cars, as they offer increased protection from side impact and are easy to add later. Sometimes called "door bars", they run along the sides of the car, typically under, or mostly under, the doors.
In addition to adding some impact protection, a well-designed set of track or door bars can also add stiffness in a part of the chassis that can use it the most (around the door openings).
Generally, door bars foul the optional door panels, so if you have these and want track bars, plan ahead in your design. These can be designed to fit around interior panels, but planning should be done with the interior panels (and seats) in place before the track bars are designed and fabricated.