The official artbooks The Art of Eorzea ~ Heavensward (which came with the Collector's Edition for HW) and the expanded book FFXIV Heavensward: The Art of Eorzea ~ Stone and Sky includes concept artwork of all the HW AF weapons for all jobs, including AST.

Unfortunately, as it's concept art, it is simply an image and has no measurements to work from, and thus is also potentially liable to degrade if you try copying the image and then blowing it up to a much larger size. It's still the best reference to work from though along with in-game screenshots (failing that, you could also try a website like FF Wiki or FFXIVDB in case they have a scan of it hosted there.).

Either way, good luck with your costume!