You can turn your sink into a fridge by wrapping your food in plastic trash bags, double wrapped is best, and putting it in the sink then fill the sink with ice. I do that when I don't have a fridge. To keep it cold during the day, fill the same plastic bags with ice and put them on top of your food in your bag.

I liked to bring wraps with rotisserie chicken, lettuce tomato and onions with refrigerated vinaigrette dressing. Healthy and easy to pack. Requires a little time at home before leaving to get it together though. I would also cook bigger meals at home and package them up in Tupperware and freeze them to bring with me on a trip. Forget the ice pack, takes up way too much room.

Invest your per diem into a Roth IRA and after 30 years it'll be worth a couple hundred k. All just because you pack your food.