I find a place on the board, which is free of holes. The best boards are the simpliest ones: old network cards, sound cards, etc. Motherboards are usually multi-layered (there are layers of copper foil inside the fiberglass), which is not what we are looking for.

Using dremel and drill, cut the horm (central part in the picture below) and make a bunch of holes in it. The row of holes at the bottom is for lightness sake as well as for epoxy to fill. I usually taper the lower part of the horn, so that the end fartherst from the pivot (with the smallest hole) is the thinnest.
The horn is glued between two ribs (dark parts in the picture).

I got this idea from Mr. Graham Woods site:
http://www.favonius.com/soaring/bagging/tail_end.htm

Hope this may be useful.
Boris