[QUOTE=aggiegrads;22051395]better yet, cut them to be under the rim, and use a stepped presta nut (like come with Continental tubes) to keep the valve centered and you would never know there is a converter at all. Very clean. The primary reason for the grommet is to keep the tube from herniating at very high pressures.
I'm starting to think the best solution for the Schrader-to-Presta rim modification might be to simply put a flat washer on the inside of the rim under the rim strip, with a hole in the washer that's just large enough to pass the Presta stem (6 mm) and that covers the Schrader hole from the inside, and the stepped presta nut depicted in your photo screwed down from the outside. Those Presta nuts can be purchased inexpensively or the LBS might actually have some tubes with those nuts they're trashing I could get for free.