These are some inserts I created for anyone making the Ironman helmet. Drill a hole matching size, print this and use it as a plug to create the detail of the screws through the helmet.
There are two items here, the screws and the jaw hinge points. You will need to print 7 screws and 2 hinges.
They are sized at 80mm so I could work with them in Sketchup, but scale them down to around 8mm for printing.