I have a back pack and put two frozen 20 oz MT. Dew bottles in the outside pockets. By the time I get thirsty they are thawed pretty good. I have 2 five gallon buckets with lids, on each side of my box in the truck. I fill them with water before I leave and drop a frozen 2 liter in each. It"s fairly cool even after 5-6 hours hunting. I take the lid off one when I get out incase a dog comes back he can get a drink. I leave the lid on the other until I get back. That way when I forget the lid is off the other and splash it out flying down the road in a hurry I still have another one. I HATE a camel pack. The water gets hot to quick and sucking out of that thing just doen't do it for me.
Oh, and you will know you forgot to empty the buckets when you stop at a red light in town.... your truck goes to rocking. It took me a while to figure that one out.