All I can think of for making the costume to be steam-punk would be to choose a rustic camouflage, like choosing a more beige color. Another cool thing to do with the outfit is to make it look more "beat up" as if you just got out of the apocalypse. Try some ripping and tearing on the outfit. Another great idea would be to make a rustic looking gun. I know of a great tutorial done by the Nerdist Channel on YouTube. Here's a link to the video-http://www.youtube.com/watch?v=v9wzwKyg3yA
They also have a few other great steam-punk tutorials if you would like to check those out as well. I hope this helps with your costume!