How about a variation on #3 ?
Sculpt, pummel, shape a mound out of oil base clay. Then 'paint' the mound with something like fiber glass resin, or fiber glass resin thickened up a bit with 'Bondo' (Bondo = a ceramic/resin auto body putty)
Then when the 'shell' sets up, you take it outside, and pour boiling water on the clay, and the clay runs out (all over your garden.)
That leaves you with a ruined garden, and an empty shell that, if you're lucky, is shaped like your currach.
Regards,
MW