Fortunately, I had a good spring to use as an example, so I wasn't "flying blind".
Measurements are quite simple and only one is critical - the straight arm of the spring is 3/4" long; the L-shaped arm has a 3/4" long extension, then bends down at a 90-degree angle for another 3/4". There is only one coil.
For the price of one ready-made spring on the internet, I purchased a 446-foot coil of .029" diameter spring wire. I think I will have enough wire to fashion my own springs for several lifetimes.
I cut off about a foot of wire so that I would have enough to grab onto and wound it around a bolt. I like to use a bolt because the threads help keep the coils even and straight.
Then, I cut one arm off at 3/4" and the other at 1 1/2". I put a 90-degree bend in the long arm and was ready to install.
Unfortunately, I made the coil too small. This is when I learned that the critical measurement is the coil section - it has to be approximately 1/2" in diameter.
Larger diameter might work, but I made the first spring with a smaller coil and the bobbin winder mounting screw tightened against the spring and would not allow the bobbin winder to swivel. It seems counter-intuitive to me, but the coil of the spring must be large enough to remain outside the bobbin winder's swivel arm.
Several more prototypes with unsatisfactory coils.
I finally found a dowel that allowed me to make the correct 1/2" coil and got a good copy of the sample spring.
If I were going to make many more, I would pick up a larger diameter bolt to wrap the wire around but I only need two right now, so the dowel will suffice.