We've discussed different mans to accomplish this a few times. Try the 'search forum' link.
My personal favorite is the Vac-U-Lock. Readily available, fits over a wide range of chastity restraints, and a broad selection of hardwear (dildo) choices. She can get exactly the size, shape, length, and color SHE wants, while you provide (only) the hip thrust. Heck, pop in an Aneros and you can get that important gland drained at the same time, without any genital stimulation for you.
Stormy Leather also sells a strap-on harness for your chin. You can penetrate her as you lick and really work out those jaw muscles!
Above all, have fun experimenting!