I bought one of those little soft coolers from Wal-Mart. It holds 2 sandwiches and a snack or whatever else. And I take 2 water bottles and fill them up half way from my water filter at home and freeze them. In the morning before I leave I fill the bottles up the rest of the way with cool filtered water. These keeps my lunch cold out doors all day!

Plus I have ice cold water to drink all day.

And about the food thief, my dad told me he had one of them at work once and he just got some jelly filled donuts and used a syringe to inject exlax. One of his coworkers went home sick that day and never returned. And food stopped going missing!