After printing part of the helmet from the design I remixed this from, I weighed the colossal support structure and it weighed in at a whopping 140g!!!! I decided to tweak the design slightly, removing the top of the helmet from the part of the shell below it, meaning it only needed a minimal structure under it to make it print nicely!
This design saved me approximately 9 hours print time and 140g of filament (as I'd already printed part 2) , but if you used this design for both part 2 and part 3, it would save you 18 hours and 280g of filament!!!
To help with scaling this for a good fit, I've also included an assembled 3D file of the helmet - simply scale this until the measurements will fit your head, and then use the multiplication factor (i.e. if you scale it to your measurements as a whole, and find it to be 1.05x scale) to scale the individual pieces, or in Cura just right click and press "Seperate into Parts" to dismantle it.
NOTE: Don't make my mistake, check that your head will fit through the hole at the bottom, not just inside!
Credit to Rafa92 for the finished photo - still haven't painted mine