Looking for design info for a shaded pole for large contactor. Everything I find is for motors
I believe that shading coils are usually a single turn that often tightly fills the slot. You may need to fill the slot more completely. As mentioned in another answer, the shorting of the two ends should be welded rather than soldered. A solder connection may have too much resistance. The shading coil shown below appears to have been made by punching it out of a thick sheet of copper. That would eliminate the problem of not shorting the ends together well enough.