So 11.4" (29 cm) is ok?
I am making a cardboard helmet based on a 3d model. It is not completely accurate, but the closest I can get with my limited skills. I plan to fiberglass and bondo the heck out of it to make it look like a vac-formed piece.
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I unfolded the file (mostly) in pepakura designer, and made it as clean as I could. The .pdo file is 17 pages that contain the base helmet, and 10 pages of greebles like lenses and vents that I didn't bother unfolding properly. The file is unlocked so if you would like to unfold these pieces, you can.
The file is here: http://files.filefront.com/StormtrooperHelmpdo/;9567343;/fileinfo.html