I'm doing the same, with a mic and the icomm/amp from tk products mounted behind the chest plate. I'd like to be fully wireless, so that is just more room I'm gonna need in my lid for items.
So, a fan (2), batteries to run them, my wireless receiver, microphone, some type of padding, and an external noise amplification system.
I'd prefer not to drill holes into the helmet if I can avoid it.
And, on too of it all, I want it to look cool. On the inside as well. Lots of trial and error and some planning... I'll post my pics when my lid actually arrives and I get started.