i did that on a friend's s13 install, but instead of a router i drilled a hole slightly smaller than the head, put an even smaller washer in and hammered the hex head in.
that way it would hold while we wing-nutted it from underneath.
on my car, i put in the child restraint to bolt it down.
sounds like your pretty much well clued and on your way...
just be sure its down securely, the more solid it is the better the sound quality.
dont forget to put some foam underneath the shelf.