What kind of flat vibrators work best for coupled sex?

My Google product searches are failing me since I don’t know how to phrase this properly. Basically I am looking for a flat vibrator that serves the same purpose as a vibrating cock ring: clit stimulation during penetration. Particularly for cowgirl it would be nice to have something to grind on that doesn’t get in the way but mostly stays in place.
I have seen some panty vibrators but they might not stay in place without the magnet holding them. Grinding pads are flat but too large to use during intercourse I think.
Any suggestions?