Someone asked me how I made my FF7 SOLDIER swords---constructed from a papercraft template---strong enough for use as cosplay weapons. So, I divulged my favorite techniques. This can work for all kinds of swords and armor, of course, not just Final Fantasy. Even original designs! Though always beginning with a shell contrived from polygons (papercraft templates) is recommended.
To make my SOLDIER swords, I flattened out several breakfast cereal boxes and traced each of the template pieces onto the brown side. (Because the waxed side is just too hard to write on, or see amongst the cartoons.) After I cut the cardboard out, first I assembled the sword in two major parts: the blade above where it will connect to the hand guard; and then the hand guard, handle, & pommel. Then I took a lightweight hollow steel rod (could use wood or any other strong light material) just slightly less in diameter than the handle, and about 2.5ft long, and inserted it into the handle through two round holes I cut in the hand guard, all the way to the pommel. I would recommend greasing the rod with white glue beforehand so that it doesn't slip towards the tip of the sword after everything's finished. Finally, slip the blade half of the sword over the remaining end of the rod and glue it to the top of the hand guard, as well as along the rod on the inside if you can manage it.
After that, I paper-mached mine for even more strength. I've found that, for cosplay weapons in general, a mixture of half water and half white glue works far better than the traditional flour & water mixture. Also, better than newspaper, use paper from an old phonebook. (It's finer than newspaper, but stronger than tissuepaper.) As you are applying the paper mache, be extremely careful that the strips wrap neatly around the sword's geometric form.
If wrinkles or bubbles occur under the paper, then when everything is dry, instead of making the sword stronger, they'll peal off easily, or even feel 'spongy' if it's REALLY bad. But, if it works out and the sword dries hard, (should be able to knock on it like a hollow piece of wood), then you should be able to sand it slightly afterward so that the paint will apply smoothly.
For my helmet and shoulder armor, I used heavier cardboard taken from pizza boxes. Wherever there needed to be a fold, I deeply scored the cardboard with a snap knife (personally I think they're way better than x-acto knives) on what was to become the "mountain" side of the fold. Same for the glue tabs, but on the tabs I had to peal away several layers of the cardboard so that, once glued in place, the two faces aligned evenly. Kinna hard to explain without a visual, but definitely a technique that works beautifully, as you can see from my unpainted cardboard SOLDIER helmet.
Whenever it is that I finally get around to making that second helmet for my brother, I'll do a video tutorial for each of these techniques. ...Hopefully...