I figure most costumes take some time.
Here is a link to some guy making an NCR Veteran Ranger armor, it goes for 14 pages with comments and stuff so I advise you read it. It doesn't seem too hard just time consuming. You would also need all of the right materials.
http://www.therpf.com/f24/fallout-new-vegas-ncr-veteran-ranger-finished-pg-8-a-101257/
It looks like he made the armor from foam, bought and army helmet, and made the mask from paper and doing something to it.