You're right, it is basically a sheet rock sandwich. I do not trust drywall anchors. You do not need to tap the plate, a standard nut and washer will work, but I had the tap but not the nuts and did not want to run to the hardware store on thanksgiving.
http://i1220.photobucket.com/albums/dd453/macdiesel/Hangerandtappedplate.jpg
The hook around the buttstock needs to be as short as possible, as you have actually pivot the weapon slightly to get it in and out. Not much, about an inch, but the first one I made I had to cut the hook back as the gun had to rotate quite a bit. This caused problems with the pistol mounts.
Also, the hook on the bottom only needs to be about 3/4" tall. This will help to get it in and out of the hanger as well.
I used 3/16" round stock with 3/16" fuel hose to protect the gun. #10x24x 1-1/4" cap screws.