Foam thickness on a helmet generally wants to be a bit thinner becuase of the angles involved and all the bends. I've seen it done with 3/8", 1/2" foam (10mm 12mm) but something closer to 3/8" or 1/4" would be nicer to work with. What have you got? And did you decide what helmet to make out of ffoam, becuase that could also make it easier or harder.