I purchased blocks of balsa from a craft store and glued them all together as shown with wood glue. The individual blocks were approximately 3" x 5" x 1.5". When the glue was cured, the large resulting block was trimmed so the final dimensions were 6" x 5" x 3.5".
I made a side-view template of Iron Man's helmet by tracing a side photo of an Iron Man toy using basic drawing tools on my computer. I've included a PDF of that template just in case you are so inclined to make one of these.