Rubber grommet?
Get one with a small hole in the middle (I've also seen some with "pizza slice" cuts) so you can get a grip on the grommet to pull it out when you need to... the small hole would have essentially no effect on the FE.
One of the papers linked to on here also indicated that having "donut" shaped wheel covers (that is, a ring with a hole in the middle) has minimal disadvantage compared to a solid wheel cover... meaning you could just leave the hole uncovered and still gain nearly all the effect of a solid cover. Unfortunately I couldn't tell you which thread this information was in.