Go to like walmart or joanns and check their costume patterns books.
They have cloaks.. And dont be fooled into thinking you could only have a piece of material hanging from your back.. Although it's more work, having the cloak full sized [So It could fall over your shoulders and in the front if you wanted, though for the costumes sake it would just be hanging behind you] gives it a much more flowing look.. I've seen examples of said theory, and it looked beautiful.
Specifically, check here: http://www.h2.dion.ne.jp/~emono.h/2006_grassland.html
As far as the poncho? Might have to be a trial and error thing... Wrap the material around your shoulders, or dress form, and have someone pin it over the contour of said shoulders. Hem the bottom, sew on the collar separate. [Rough idea for me.. I dunno, seems to be working so far.]