I've actually been looking into this option too, and it's a possibility. But a ready-made costume would be a lot easier, probably look better (given my skill set when it comes to sewing), and would be well worth the money (assuming it's in the same range as the Spirit Halloween price).

EDIT: I think purchasing one is looking unlikely, though- I keep checking back and everywhere I look, the thing is well and truly out of stock except for kids' sizes.