Well I finished the install of my undertail today. Like I said before, removing the back half of the original undertail left me with very little to mount the new one. Like you said Zippy, these parts aren't manufactured to very high specs, but there was enough of a flange around the whole part to use some very small self-tapping pan head screws at four points and just screwed it to the left and right tail halves from below. You can really only see the screw heads if you look underneath, and I'll probably just paint them to match the body color.
Now, all I need to do is get some aftermarket turn signals, or a turn signal eliminator - the bracket supplied with the undertail doesn't work with the stock zx7's gigantic signals (wish I would have thought to check that before I drilled for to mount the dumb bracket).