Jeff, you might consider doing what I have done. Go to the local ACE hardware and buy 1/4" OD aluminum threaded bushings. These come in a various lengths, so buy a slightly oversized length. Look for 6-32 threaded stock items. Epoxy these into the control surface and voila, instant hard point that has a built in thread. There's no need to cut these flush to the surface. You can make a very neat installation into the surface with care.

As Steve stated, don't use all-thread material for horns. But screws will work great. Following Ron's note, to bend them easily such that the horn is right over the hinge line, use another bushing on the screw, and you get a nice handle to work with. It's very simple and effective.

MattK