You might think of trying to start with a pattern for one of those western-style shirts with the cavalary bibs (like this one: http://westernandwildlifewonders.com...f10d4267db5063
) to get that bib on the front that Amon has. Just leave the collar off and attach a hood instead (pretty much any hoodie or hooded jacket pattern will do for the hood pattern).
Rather than attaching the tails separately, you can just extend the patterns for the shirt back to create the tails, so it will be all one piece. It looks like the bib on the front also extends down as a skirt below the waistline - just lengthening a cavalry bib pattern would give you this look fairly easily.
The mask shape isn't TOO unusual (not like the Blue Spirit, anyway) so you might even be able to find a plastic base you can paint.
Pictures and tutorials from my costuming portfolio at Syagria's Cosplay Site!
Star Trek Original Series Uniform!
Original Creation for AB13
Most Recent Con:
PAX East, Anime Boston 2013