I know some people have made covers for S wheels before the original aero wheels were a thing, but haven't seen anything for the X. However, I would take the stated 10% supposed improvement with a grain of salt. That is most likely under ideal conditions at steady highway speed with no wind. Lower speeds, stop and go traffic, cross winds, etc. will lower or altogether negate the improvements. It is hard to estimate what the range improvement would correlate to on the X. The 3 is already lighter, more aerodynamic, smaller tires for better rolling resistance, etc compared to the X. These issues with the X may mask any efficiency gain achieved by wheel covers.
Worth testing and experimenting with. May want to search over in the S forum for some posts.