Make a hole in the front of the helmet the size of the servo motor. There are probably more elegant ways to do this but I used a drilling machine an drilled small holes along the outline of the servo motor and then used a knife to cut it out.

Then take the plate from the robot arm and drill holes in the helmet that co-align with the holes of the plate.

Mount the servo with the motor mount facing down and put the acrylic square on top of it. Use the two long screws and the two nuts in the two upper holes. I had to cut out some of the insulation of the helmet to be able to screw them in. Then take the two shorter screws and put them in the two lower holes. It's important that the head of these screws are on the inside of the helmet as they otherwise will be in the way for the robot arm to move.