Ok, another good find. I realize that after building a Fett, I'm now focusing on how to stay in the suit longer by increasing comfort. You never know how uncomfortable the suit really is until you've had it on for 3-4 hours. The main thing for me has been the jetpack...I've got a bad back as it is, so after 2-3 hours my shoulders just burn up. But another key point has been the helmet - seeing well, breathing well, and now staying cool. I've seen people look into ice packs before, or personal cooling systems, but they all have high profiles that may bulk up the costume or not fit properly.
I found a great site while researching but it's from Australia: Neck Tie Coolers, the best personal body coolers, help beat the summer heat.
Essentially, I was looking for something that I didn't have to put in a freezer before wearing because realistically, I'm not going to lug a cooler around with me in addition to my gear. So these really stood out as they were a gel that activates by just being submerged in water. But...they were in Australia, so I figured there must be something local.
I went to a local fire safety store and was able to find these headbands that do the exact same thing. They were a whole $6 each, and are reusable:
I tried it on for a test with and without the helmet, and it works great. They are rated to be good for up to 5 hours, and just need to be resoaked to be recharged. They're low profile so it's not bulky in my helmet. I might even wear one around my neck.
Plus they make you look like a ninja.