Also through experimentation, I made a forming block. I used the same block for all the horns. Once I made the holes for the dowel pins I used, this block took on a PG-13 rating.
The blank is then sandwiched in between the forming block and a back-up block in the vice. The dowel pins insure it won't move. You must also insure that you consider the radius of the bend and any spring back that will occur when forming when making the block.
I used a ball peen hammer, and a piece of Oak for a striking device. The oak helps shape the aluminum and eliminates hammer marks.
Here is a finished horn set. They are riveted to the top a each aileron and flap and attached to the controls with a clevis.