Get a real vibrating cock-ring from an adult store.
As far as stimulation during sex goes it's really the only one that works during actual penetration without having to adopt special positions or hold something in place.
The thing is, you haven't actually stated what kind of stimulation she wants. Is it the rubber finger kind (ala cock ring w/ ridges), or vibration on her clitoris, or...
(No, I'm not a woman but I tend to know entirely too much about sex toys...)
If it's vibration she wants, then you can use any reasonably sized vibrator to hold against her. Again, it tends to limit positions for you due to the logistics of holding something in place during sex. Let alone vigorous sex. Hitachi magic wands work, but they're so bulky that it could interfere with your ability to thrust hard without hurting you or her.
Pretty much sounds like a real vibrating cock ring. Take her, go look, pick out something that will work for the two of you. I prefer the non wired versions as the wires just kind of get in the way.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."