So this is a project that I kinda put off but here it is finally. It is modeled by AceHigh and unfolded by Jason02.
Its not quite built entirely but it only has a few more pieces.
This really depends on your helmet. I built mine out of 110lb cardstock so it is pretty sturdy. I then applied 2 coats of resin to the outside. The cheek pieces and the nose were the only places I reinforced with fiberglass cloth. I did that mainly because they weren't holding shape very well. I found that when you are working with the cloth it is best to put some resin on the place you are going to apply before you place your cloth down. It helps by giving a more solid connection. Some people will put cloth around the opening of the helmet.
If I remember correctly I found the file on the RPF. I believe if you look up leonidas 300 pepakura you might find it. If i find it soon then ill put a link on here.