Make sure there is no water in the hulls, or other areas (is your mast watertight?) and cover the hulls at the very least. The freezing temps alone should not hurt the boat, as long as it is dry. The other thing to consider is snow load. Even if you cover it, that could create a problem w/ snow load on the boat and or tramp. You may want to remove the tramp, and wrap the hulls individually. That way the weight of the snow won't really be a factor. Allow it fall right in between the hulls/crossbars.
I don't know what is involved in removing a tramp from an FX, but this is my suggestion anyway. Just keep the snow load off the tramp whatever you do.
I have also seen people drape big heavy tarps over the mast (as on trailer), creating sort of a 'tent' like structure, but I don't know if this will work in areas of really heavy snow and wind. The winter we had last year in New England would have been way too much snow for such an arrangement.
2007 Hobie Wave