I've used the kind you soak in water. Has to be dry out, these probably don't work in the midwest. They worked pretty well, kept the body temp down.
A better idea may be to see if you can do your attic work at 5-8 in the morning. I know of a few HVAC companies that do this, and the early riser customers don't mind it a bit. Actually, they like it because they don't have to take time off work to let the guys in.