On a LOT of the newer Great Planes and Top Flite models there is a 1/8" plywood plate on the bottom of the control surfaces. You use a 1/2" #2 sheet-metal screw to mount the horn. The horn is then removed, the holes are hardened with thin CA and the horn is reinstalled.

Many of the planes I reviewed in the last few years use this method and it works very well.

Here are a few of the reviews where you can see this method:

[link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=1134]Top Flite P-47[/link]

[link=http://www.rcuniverse.com/magazine/article_display.cfm?article_id=870]Top Flite Cessna 310[/link]