Based on Sunday's design found here: http://www.thingiverse.com/thing:717766.
I sliced into 2 parts for easier printing and fixed stl file. I added foam padding inside and a chin strap. Pictures are of the actual helmet and my Captain America costume at the Avengers Age of Ultron premiere with the rest of my family.
Its current size fits 24" circumference head, measured at the brow line.
I printed it in HIPS and glued both halves together with plastic epoxy.