i have the same helmet. It fits like a glove. But some Melon Alleycats i had didnt fit with it. The arms were too long and rubbed on the arch that comes down the back, behind the ears (if that makes sense).
I think i need glasses with a rounded/hook arm rather than straight ones.
But i too have an odd size head. The POC helmet sits on my head, not dissimilar to that pic. And my previous Giro Montaro did too.
I tried a MET and a Troy Lee, and whilst they felt like they fitted, from the side, the gap between the top of my ear and the helmet was absolutely massive.
So umm… yeah, not really sure what the solution is?! cut the arms off and glue the remaining shades onto the front lip of the helmet?!