I have just quickly measured my cape it is put together from four shaped panels plus a collar.
There are two identical front panels measuring 48 inches 46 inches 40 inches in the shape of a triangle the longest measurement being the centre seam.
The lower back panel is bottom 80 inches sides 20 inches top 38 inches.
The top back panel is 38 inches sides are 20 inches top 12 inches
The collar is 24 inches and 2 inches deep.
The various panels are shaped or tailored in a complex fitting.
There are also various tabs two at the front to slip your hands through and a loop at the back to put a belt through.
Not many people use cycle capes but I find that mine is brilliant. It keeps the rain off the back and lower body arms hands and legs in still or even moderately windy conditions. It can be a problem in very windy conditions. In a head wind it is of course not very aerodynamic.
You would probably need a very skilled tailor to put together something that works, in that case I would guess it would be cheaper to buy a ready made version.
For example Caradice see here
http://www.carradice.co.uk/index.php?page_id=product&under=other&product_id=66
The biggest advantage of the cape are those humid wet days where you stay dry from the rain and the airflow under stops you getting wet from sweat.
