Thanks for the help guys! The helmet is a T/MC, and I trimmed and assembled it myself. That's an interesting point that assembly determines the size. I'm used to my MR helmet, so I was unsure how small the hole was meant to be. Thinking about it, I once tried a friends RT-Mod and couldn't even get it on my head
Perhaps it's my head that's in the wrong place and I have to adjust the padding? Because as it is, the vocoder really digs into the my throat, but there is a lot of room at the back.
Thanks for the photo, that helps a lot!