For the Corvette and Grand Cherokee, I have used the rubber floor mats and cargo liner from WeatherTech
http://www.weathertech.com/
While priced on the higher side, quality and appearance is excellent. Not the same appearance as carpet, but an alternative.
If rubber mats are used in place of carpet mats, the sides on some of WeatherTech mats can be trimmed, score lines, to fit. Reply from their Customer Services Dept suggested the following as an approx fit for the Cobra, but clarified on the need to be verified for each unique application. There are provisions for hold down tabs, i.e., use of factory retention devices
Model W3, Universal Rear, LHS mold 1000 and RHS mold 1001
Model W47, Designed for 2003-2007 Cadillac CTS, LHS mold 1000 and RHS mold 1081