For full rotation (360°) you need a mount that will hold the whole unit far enough away from the mast
tube so that the blades will not strike the mast when rotating, but also high enough above the bottom portion of the mount so that the blades will not strike the mount itself.
Generally, you will have to design the mast mount yourself or have a machine/welding shop do the job. And then have it fabricated by the welding shop from marine
tubing or shaped tubing.
To meet the requirements, others I have seen use a "tripod" mount system where two arms, one mounted on each side face of the mast come out horizontally the required distance mentioned above, and a third arm comes up at an angle from lower on the mast to join the two horizontal arms. Then a length of proper sized, thick wall tube is welded to the tripod and rises vertically from the apex the required distance and the wind generator
is mounted on its end.
The "long" arm extending out horizontally requires the bottom angled leg of the tripod to support the weight of the wind generator
No matter how "strong" you make the braces, their long "arms" will allow the wind generator
to "twist" or move laterally and fore and aft. The "thinness" of the mizzen mast is a major problem in stopping lateral sway.
For wind generators like yours a "mast head" mount is the best answer. My Air Marine
is mounted on top of the mizzen mast with a proper length of aluminum
tubing that allows 360° rotation. The bottom the mount tubing is attached to the masthead cap.
The higher you can mount wind generators like yours or any with 3 or less blades the better performance you will get. For units you want to mount lower, I personally, recommend the wind "turbine" (5 or more short blades). The length of the blades is the limiting factor determining how long that "arm" extending out in front of the mast has to be.
An alternative is to not allow the unit to rotate 360° by installed "stops" to prevent the blades being able to contact the mast or the mount. Then the mount can be a lot smaller. Since sailboat at anchor
are normally facing into the wind or at least have the wind forward of the beam, this restriction on swiveling rotation is not a problem.
In the photo
below you can see the "restricted rotation" mount low on the mizzen mast and the masthead - full 360° rotation mount on top.