You could do it, but you'd need two things:

(1) a pair of matching coats so the cape was exactly the same material.
(2) a REALLY heavy duty sewing machine.

A friend of mine went down to the gament district and bought enough fabric for two overcoats. He then went fishing on the internet and located a tailor in Hong Kong who agreed to make the garment. (I think it was Ricky Bo at the HK Hilton.) The fabric, and an overcoat that was the exact size required, were dispached to the former Crown Colony and after about six weeks his Inverness cape arrived, complete with Burbury check lining! The final cost was something of a state secret, but I'd guess it was probably around $300-- or what you'd pay for an ordinary rain coat at your local men's store.

I've a friend in Ireland who has an Inverness cape (actually it's an Ulster) that was made for his grandfather's chauffer. Absolutely an amazing piece of tailoring, with sleeves that unbutton from the shoulder and can be removed or put back on as the weather dictates.

Of course, there's always the ubiquitous umbrella...