I need strong magnetic fields from such windings.
The magnetic field lines don't always add because of the air spaces. (The insulating coatings on the wires will have to allow magnetic flux through easier than air; greater permeability. Wrong! This means shorting the flux lines inside the coatings.) The insulating coatings should have less permeability to encourage flux lines in the space around the coils.
Would the field be even stronger with a hollow rectangular tube?
