Like Big Max said, use a piece of wire, I ground mine down like a screwdriver, it will go right through the balsa, and the long drill will help you see the angle you're drilling.

But the real trick is this...

When mounting the horn, get one screw started, but don't tighten it! Then start the other one while the backplate is still away from the surface. Now, if they don't line up, you have room to squeeze the screws together, or pry them apart, or whatever. Once BOTH screws are started, go ahead and tighten them down evenly.