More construction info... (thanks SN)

Use the 60 x 2 tube, like Gizmo said - it is literally "available on every street corner"

Not shown in the plans as these improvements were developed at a later date :

: mast step can be vertical [instead of the 10 degree rearwards lean] - this has proven to work very well with unmodified windsurfer sails.

: weld a plate over the rear of the chassis spine tube - then the tube is less liable to tear at the axle tube welds due to twisting.

: run a steel strap [flat bar 25mm ish x 5mm ish] down one side of the mast-step, underneath the chassis spine, and back up the other side of the mast-step.
Stitch weld the strap to the mast-step and chassis spine, not a continuous weld.
This will prevent the mast step welds from fatiguing and tearing apart due to stress.