Same thing is required for keys on some fore-ends. Basically you need a nail header-like device that accepts the spring blank but is angled on top. 1" square steel stock will work if you can get the slot cut in it. Then mill, grind or file the surface of the form to the right angle. Get the spring blank orange hot, drop it into the slot, and tapitty-tap with a little ballpeen hammer to re-align and widen the head. If you grind back the form beyond what the spring head covers, then you can approach it at good angles with the small ballpeen hammer. I guess if you use mild steel for the spring, you can cold form it and not worry about the heat, which disappears pretty fast.
I can't draw diagrams but maybe you can get the idea.
Of course all this has to be done with a rough blank as the first step.
When I needed to make a key for a double barrel shotgun, I cheated and quickly made the "form" of 2 angled pieces of steel and just held one higher in the vise and clamped the "key stock" between them. I had to be careful with the peening and it required some cleanup, but it worked out fine.
