Control Horn Jig
It's easier to pre-form the .025 sheet steel on a piece of 3/8 in tubing.
Then form it on the jig. If you clamp the sheet metal firmly at the bottom center of the jig (on the streamlined tubing) it will hold the sheet metal in place while you curve and flatten the spine against the 1/4 in tubing. Lots of soft taps!
Here's the spine after some tapping.
When the spine is formed, run a bunch of tack welds along the edge, remove the clamps and weld. Here's a picture of the horn after welding and a bit of grinding and sanding. I'll smooth it out later.
Leave the horn unwelded at the top. We will insert a piece of .125 steel there later to make the fitting that holds the shackles. See below.