i would just cut the opening bigger after i hardened it, or you can cut it now make sure it fits and then add sticks from side to side where your ears come through the opening. widening the opening slightly more and then resin with the sticks in place to hold the shape. over all you will have to experiment and find out what works for you. take down notes. i would make a list starting from top to bottom and write down i.e the helmet used, scaling used that worked for you. then repeat for the rest of the armor if you are going to build a complete set.