I took 1/4-20 shoulder bolts, cut the heads off them, and welded them to the tunnel threads up. When done right, the shifter sits on the shoulder, and a few nuts to hold it down. No need for a second person. Like this, you can position the shifter how ever you want. Just cut to the right length for the placement and weld.
Someone mentioned being carefull when drilling through carpet. I have a trick for putting holes in carpet. Find your location, then poke it with a hot soldering iron. Melts a nice clean little hole. Off coarse, have some water around just in case, but I've done over 15 holes without incident.