Many thanks to y'all for your help. I figured an internal fan would be a little overzealous, but thought I'd ask anyway.
If anyone else reading this has sobering to contribute about fans, please do!
Grif, did you have to make the earholes yourself? If so, how did you do so without compromising the helmet? I imagine some gentle carving and smoothing?
If these questions are tedious, I apologize.