Good to "see" you here, momorris. What vehicle are these wheel covers going on? That info might get you some more specific tips / pointers here.
Smooth wheel covers are definitely a good project, judging from the gains others have reported. I'm partially done on mine now. I can't imagine taking the time to do them in fiberglass but who am I to say it's not worthwhile?
Most guys seem to have used plastic serving plates or stamped metal pizza pie pans. I'm using perforated aluminum sheet with a stiffener on the back side, but I can definitely tell you that method is not quick.
If you go to the project library:
EcoModder Project Library - EcoModder.com
you'll find a few threads on smooth wheel covers. See the section "Project Section: Aerodynamic mods - fabrication", and you'll need to scroll down 15-20 threads there (currently) to see the ones on smooth wheel covers.
Here's a thread I've based some of my project on:
http://ecomodder.com/forum/showthrea...orn-14306.html
That wheel cover should be suitable for most wheels, the attachment method is specific to Honda HX and VX and maybe for alloy wheels in general. Worth reading though.