The SL-C has a roof scoop and the basis for a duct on top of the roof, that can be used to direct outside air to the interior, or to the engine compartment. However, the factory does not provide a way to seal the roof scoop opening, or seal the upper duct (i.e., the underside of the scoop in the roof). Without such sealing, outside air and moisture will enter the interior. In addition, absent such a seal, hot engine compartment air can migrate to the interior more easily, especially at idle when stopped, so the use of such a lower duct can also help to reduce interior temps.
Luckily, another SL-C builder has designed and fabricated a lower duct piece in fiberglass that fits very well, and seals the interior against air and moisture that enters the roof scoop opening, so that outside air and water can't penetrate the interior (unless you want to get outside air, which is discussed below.) Air from the roof scoop can then be safely ducted to the engine compartment.
The scoop opening in the body is typically uncut, or rough-cut from the factory, and needs to be opened up if it is to be used to actually duct air. Use a Dremel or a file and carefully open up the scoop opening, using the aftermarket lower duct piece as a guide.
Normally, a grille of some sort is used for both appearance and to filter out large objects. Most builders who use a grille there use the same material to fabricate grilles on the side openings just ahead of the rear wheels.
For street cars, the opening can be blocked off if engine or interior air is not needed. If the roof scoop opening is closed off (via a user-fabricated fiberglass cover, for example) the lower duct piece discussed here won't be needed, of course.
For race cars, or street cars who want to use the scoop, there are several options which are discussed below.
A popular use of the scoop and duct is to direct air to the engine compartment, either for cooling, or as a source of engine intake air, or both.
The duct opening has more than enough area to support any power level, even if it is the sole source of engine intake air.
Interior Fresh Air
Because there is typically no source of outside air into the interior aside from leaks, some builders use the scoop and duct to allow fresh air into the cockpit. This is typically accommodated with helicopter vents like these installed in the interior roof panel. Many different styles are available, but be sure to get the kind that can be opened or closed, to control airflow.
Sealing the roof scoop and duct
As delivered, the scoop opens into a raised hump (the "upper duct")in the body. If the opening isn't sealed, rain can come in through the scoop, and leak into the interior. As discussed above, there is an aftermarket lower duct piece that seals against the body in the front by the scoop opening, and runs along the car to direct all air (and water, etc) to the engine compartment.
Such a duct is very useful for the street, as it allows the roof scoop to be functional, does not allow outside air or water to leak into the interior, and provides engine intake air, or cooling air for the engine compartment.
If you don't seal the roof scoop opening shut, you'll need to seal the rest of the duct, or you will have water leaks in the interior. Note that the optional roof panels (as part of the tub or as a standalone piece) don't adequately seal this area, nor were they intended to do so.
Where to get a lower duct?
SL-C builder Wayne Marov designed and fabricated a great lower duct piece from fiberglass, and made a mold. Jack Molleur has the mold and can make these ducts on request. Jack can be reached at firstname.lastname@example.org . As of late 2015, the lower roof duct is $140 plus shipping.
They are reasonably priced, fit well, and offer the advantages discussed above. Every street-driven SL-C should have one of these, or something similar.
The lower duct can be installed by using fiberglass resin and cloth, or by the use of 3M or similar adhesive to hold it to the underside of the body.
Because the duct has no flanges along the long side, it's easy to adjust the height by careful sanding of the part. The part is a very good fit to the roof scoop opening, but that area is complex, and you may need to do a little sanding or grinding to get a perfect fit. Always dry-fit first.
An alternative to adhesives or fiberglass is to use Dynamat or similar material to make flanges that attach to both the duct itself and the body. This approach makes it less likely to create a mess because of dripping adhesive or resin. It's also faster, though usually at the cost of increased weight. Be sure to clean the duct and the body to promote adhesion no matter how you attach the lower duct piece.
Whatever method you choose, be sure to seal everywhere around the scoop, and the sides, to prevent moisture from leaking into the interior. Even if you have used adhesive for the duct, the use of tape or Dynamat can be helpful to be sure that all possible leaky areas are sealed. Using tape or carefully cut pieces of Dynamat, you can also add insulation, so engine compartment heat is harder to flash back to the interior.
Don't just glue the duct to the body without checking for fitment, specifically around the other parts of the car. To get a perfect fit, you should install the duct with the body mounted, and the roll cage installed where you will place it. You should also install the rear bulkhead panel if you are using one (one is standard with the tub option, or another variant is available as an option with the older interior pieces.) That's because the duct fits between the two bars in the cage, and must also be aligned with the rear bulkhead panel. Don't make the mistake of just gluing it up there without a clear understanding of how the rest of the car fits around it.
Assuming you use a grill in the roof scoop opening, you'll likely need to fabricate it and insert it in the opening before you place the duct in place. Do a dry fit before you open up the adhesives or begin to mix resin to be sure the grill fits perfectly and allows the duct to seat properly to insure a perfect seal.
Clearance for overhead monitors, switches, etc.
Even with the duct installed, there is room for overhead monitor and switches. Elsewhere in the wiki are pictures of a couple of different monitors and switches in Will's and Stage7's cars, both of which have full interiors, and these ducts. But you must plan for these, and make sure that all the pieces fit together ahead of time- don't assume everything will just slot together; You need to verify fitment before you commit to any specific setup.