My tripod bag for my camera is the same shape/make. So we're going to use that as a reference and just make one out of either a heavy fabric or nylon, with the braided nylon straps. That, at least, should be easy.
I'm having a hard time getting the buttons. I bought all the silver buttons BOTH of my fabric stores around here have (they only get 3 packs in at a time - I bought a total of five) and I still need a bunch. They have 12 on each jacket (6 down the front, three on each sleeve cuff), then Yukio has a total of 18 on his coat. WRY SO MANY BUTTONS?!? >_<
I've decided to go with the anime references (cause we plan to enter ours in a masq and they prefer one consistent reference) rather than the manga.