I just want to know it making some type of fin out of a material more rigid than rubber will work.
That's a good question. Fins operate like propeller blades, twisting as they radiate out from their root. But unlike most propellers, Mirage Drive fins oscillate back and forth, so their construction must be flexible enough to reverse the direction of twist without material fatigue while providing a variable rate of twist for optimal performance at different boat speeds, cadences and acceleration loads. Turns out rubber-like materials fit the bill very well.
Hobie has experimented with other materials, but without the efficiency or durability of rubber so far. They obviously continue to improve fin performance as well. Near as I can tell, Turbofins are currently in their fourth generation, as follows:
1. original Turbos
2. shortened mast by 1/4", improving twist rate
3. changed outer black rubber for better durability -- and performance
4. changed inner gray rubber for better kinetic energy storage (performance)
Of course they all look very similar so it's hard to tell which is which (except by date of purchase). Performance gains have been subtle -- most casual users wouldn't notice.
And as far as a sail kit, that is only an option if you have wind.
There are some after market sails that will fit any kayak, but they have little or no upwind capability. Some people have made their own out of a variety of materials and masts with varying degrees of success.