This is inspired by all three of the sources listed, but initially by bandit-ed's design ( http://www.thingiverse.com/thing:442460 ). I wanted something that would print without scaling on my tiny printer (120mm print bed). All thee of the "sources" were inspiration, but other than some measurements on bandit-ed's design, this was made from scratch in Sketchup by me.
I moved the screw holes closer to the ends for easier access, thickened up the bottom of the arch to add some stiffness and strength, and shortened the arm, as it doesn't feel like the length of bandit-ed's design's arm is needed for holding the helmet.
I'm still learning Sketch-up, so I simply missed some bevels where they were easy (oops), and didn't get the inner holes beveled.
In my strength testing on a previous iteration of this design (without the counter-sunk screw holes), the weakness is around the screw holes. Regardless, there is minimal flex until substantial pressure is put on the end of the arm.
This produces a final object that is approximately 90mm tall and deep, and 16mm wide.
Monoprice MP Select Mini 3D Printer
20% triangles, alternate extra wall
Prints flat on the best without any need for supports.