I use (used) this one from ebags:

eBags Professional Series Crew Cooler > Travel Coolers > Travel Accessories - eBags

It works pretty well, I just put some ice in a ziplock bag each morning and it would keep things cold all day. Four days of food stuffs it, but it fits, and it has other non insulated pockets too. I liked it because it doesn't look too much like a lunch box, and I have seem other guys with it too.