The most critical difference between the HALO armor and the Storm Trooper armor is that there are remarkably few games featuring storm troopers, and therefore, a remarkably small number of existing 3D models to work from. This is, for the most part, the first step in "ripping" a model to be turned into a papercraft design.
IMO, while Pepakura is an ideal method for rapidly prototyping an object to existing specs without a 3D printer, the physical characteristics of such a system are less than ideal for anything much more involved than display, costuming, and posing heroically at your choice of convention around your favorite developer's booth.
That being said, Pepakura offers a way for individuals to bypass a lack of fine dexterity, 3D spatial sense, and lack of fine shaping and sculpting tools provided they can follow complicated directions and cut neatly instead.
Assuming you're looking for costuming (hence the word "costume" in your post), you probably aren't exactly looking to do, for example, a foam-rubber-reinforced thermoplastic cast designed to survive you using it to smash through a doorframe, but your options are still limited.
There are three things that an armor set can be. Good, cheap, and easy. Pick two, and that'll tell you what your best option is.
