Not sure what C3s come with for a spare, but C4s come with a space saver. Maybe you could test fit a C4 space saver wheel to see if it works. Stud pattern is the same, offset is different but wont make a difference on such a narrow wheel. As long as it doesn't touch the calipers, you'll be OK.
Otherwise, Commodore wheels can be bolted on (not sure about brake clearance though) in an emergency but are not ideal. Commodore stud pattern is 5/120, Chev is 5/120.6 (or 5/4.75") so use a Commodore rim only as a last resort.