The helix is used to find 6 intersection points of the corners of the square extrusion, for one wire turn.
The radius of the corner of the working square needs to be enlarged from the ferrite measurement, by the amount of the wire radius.
Then did a polyline connecting the 5 points of one coil (Plus extra line to point six for the fillet by the 5th point). The helix is deleted.
Then did fillet radius.
Also moved one tiny fillet radius from the end to the beginning of the one turn of the wire.
Just did one turn of the coil, then used Transform Array Dir, to copy to the straight square extrusion for the solenoid.
Also did a fillet on the original smaller square, at the same radius. So there may be a slight gap between the ferrite corners and the wire.

So there was a whole lot of fiddling to do :-) , but good moi practice.

Unwrap was used on the center radius of the ferrite core to get the proper length for Flow.