Trans woman (so your milage may vary) but hitachi multiple-orgasms are fucking incredible, and completely possible. Just keep it pressed on your cock and ride it out until you can't anymore.
My "position" when soloing: I hold the head of the hitachi up against my frenulum with four fingers, with my thumb pressed down over my shaft (opposite side of frenulum). The hitachi itself is resting side-on (most of it on my hip) so I'm effectively pressing my tip into the side of it. Thumb applies pressure, grip prevents it slipping, vibrations are hardcore.
Although, I do want to try the cock attachment silicone thing.