As promised, this is the follow-up to my Cooling Vest Instructable. The Vest was to keep your torso cool while working outside or in a hot area, but did nothing to keep your head cool in the same environments.
Step 1: Create the Cool Gel
Just like the Cool Vest, you use the unactivated lining from adult diapers. Mix the lining with water and alcohol to create a gel that, when frozen, is slushy. Only this time put the gel into Snack-Size plastic baggies and put the baggies in the freezer. Double-bag the baggies to prevent leaking.
Step 2: Attaching the Cool Pack to the Hat.
Create a pouch for the Gel Pack out of the same material used for the vest (breathable, but somewhat waterproof) with a Velcro closure.
Add the "Hook" side of a Velcro patch to attach the pouch to the hat.
Step 3: Attach the Pouch to the Hat.
Sew a "loop" side Velcro patch inside the crown of the hat using a Zipper Foot on your sewing machine.
Step 4: Go to Work...
After the pouch is secured inside the hat you might have to add or remove Cool Gel from the pouch until it fits on your head. As with the Vest, you can put the entire hat in the freezer while you're resting between chores.