As I write this the weather is getting warmer. My son prefers to take a packed lunch with him to school and his favorite sandwich filling is meat. There are no refrigeration facilities at school so his lunch has to for survive several hours before he eats it.
In this summer it is very important to keep food, especially meat, cool until lunchtime. He loves cucumber too and that tend to go horribly soggy if it's not kept cool.
So it's time to find a solution that will keep him lunch cool in warmer weather so he can keep having the foods he likes.