Smarty, Smooth-on makes a liquid plastic that is flexible after curing.
Its called smooth cast 45D. You mix it and pour it into your mold, then 30 minutes later when its cured, its still very flexible. It does harden over time, but not to the point of complete rigidity. Thinner pieces are more flexible.
Plasti dip is great too. But thick and difficult to spread even, esspecially if you want a form-fitting cape or coat.
Also try flex- sculpey. Or sculpey flex... whatever its called. Its also flexible after curing.
This is the easiest way. But you have to sculpt the piece, not cast it.