A ferrite core is prefered. A metal nut doesn’t really work. Magnetic property is just not the same.
More turns equal more Henry value of the coil.
But given the size of the hole, how to calculate roughly how many times a wire of given size can pass through it?
So first, we need to convert each round wire into a hexagon in order to fill up the gaps in between them.
Each hexagon will be composed of six equilateral triangles.
Each equilateral triangle will have a height of W/2.
The area of each equilateral triangle is then calculated.
Multiply the area of equilateral triangle by six will be the area of each hexagon.
The area of the hole H will can be obtained easily.
Divide the area of the hole with the area of one hexagon will give you a rough idea of how many wire can pass through said hole.
The thickness and the height of the cylindrical core will tell you the length of each winding. Multiply this with the number of times the wire can pass through will produce the total length of the wire needed.
But with more winding, the length of the wire each turn will increase. So you may need to take this into consideration if there is a very big hole to be filled.
Again, this is just a rough calculation, accuracy is more than enough for small projects.