Charlie,
Had not seen this before and not sure exactly what type the chevy has, but Speedway has some loose caps - http://www.speedwaymotors.com/Weld-R...-Cap,4555.html
If they are the type that are held in by the threaded zirk, I would think you could make one from some thin plate, or maybe drill out a quarter (or a larger diameter foreign coin) and file to fit?