Girl on Top
Have you ever heard of a "strap-on pussy" of sorts? I keep lamenting that when my boyfriend is on his stomach and I'm lying stomach-down on top of him (or standing behind him), there's nothing all that stimulating that I can do to him with the power coming from my hips (and pretty much no motion required from him). I could reach around, of course, but it's just not the same as the hands-free motion (free to grope, free to stabilize, free to grab on to the hips like handles) he so often gets to play with from behind. I suppose it would look something like this [URL for drawing]. It's 2007. Someone somewhere has to be selling a rig like this, right?
The drawing you sent me didn't come across legibly, so I will have to answer based on your letter alone. If I don't "get" what you are looking for, please feel free to write in again!
But I believe your boyfriend already comes equipped with the device that you need. It's called his asshole. With a little bit of lube, some preparation with a finger or two, and a smallish-sized dildo, you can use your hips to give him quite a ride. His prostate will certainly thank you for the massage and release. If he doesn't like penetration, get a harness and a dildo with a vibrator in it. You can find harnesses that will allow you to use a vibrating egg against your clit or inside, while you work on his junk with your vibrating magic wand of female power. You may have to arrange his genitalia so that it's in contact with your thrusts, but this is not difficult for most guys. This way you can lay on top of him, he can enjoy being somewhat passive, and you can ride him while you work out your own orgasm. Sound like fun?
If what you are really looking for is a strap-on pussy, they do make artificial vaginas, and I suppose you could arrange one so that you could penetrate it while he holds it between his legs. A strategically placed vibrating egg at the back end of the plastic pussy would give him some stimulation. If he has cross-gender fantasies, this might be ideal.