I use rear hanging brackets all the time with undermount soft-close slides. I use left right stretchers that have the bottom of the stretcher locked to the top of the rail on the faceframe that the drawer will go into. When I mount the slides, I just use self tapping screws in the rear bracket. I space the slide from the inside of the cabinet side using simple MDF spacers. Seems to work. There are most likely much more efficient ways, but this seems to work for me.