I've used the Ozark Trail 20 oz. insulated tumblers for a couple months now, and these things rock!
http://www.walmart.com/ip/Ozark-Trai...mbler/49772705. We can get a whole day of ice out of them. The only caveat is that the lid, which is clear, needs to be covered. No matter how hot the corner of my helm gets, the ice stays solid, but I think the clear top lets the sun in too much.