If you lack walls to secure your food, try one of the on-site storage vendors, for probably $75/mo you get a 5x10ft steel storage box dropped to your worksite.

I covet (but don't own) one of those steel storage lockers you see around public playgrounds and soccer fields, where the soccer associations keep their materials to dress and maintain the fields. Usually very secure, also very expensive, $500+ when I looked into them. Although Home Depot sells a smaller version for on-site tool storage for ~$240.

Any of these wouldn't look out of place on a construction site.