Hi BW,
I decided against a cooling fan in the helmet. I tried one on the back and found it to be way to noisy (even though the fan seemed to be silent, once it was installed on the helmet it was loud). Instead I installed a medium sized CPU cooling fan inside of the jetpack. I made a funnel and attached two hoses to the funnel and ran them to the helmet. I put one tube on each side of the front visor, near the corners of my mouth basically. So actually as a replica of the Boba Fett helmet, that changes things. But all my friends and family said it looks really cool. It keeps my face shield from fogging up and helps me breath better. Cool air blows up my face, it works really well. And... you can't hear the fan. I powered the CPU cooling fan with a single 9v battery. I installed a small swith onto the bottom of the jetpack that I can reach with my right hand. I can pull out the air tubes then remove the helmet. The air tubes did hinder the movement of the helmet, just a little bit. So I added a little rubber to the inside of the helmet near my check bones, the helmet fits a little snugger and allows for full movement. Good luck.
-Indae007