How do you carry lunch to work alongside a full backpack?

[Homeiswherethedogsare]

I commute with a backpack but it's usually full so no space for a packed lunch.
Need to start bringing my own food to the office and just wondering whether I need an insulated lunch bag? Or just a tote to carry on my shoulder as well as my backpack? Or just find a way to chuck it in in a plastic bag to avoid leaking?
What does everyone do?

it depends what else is in my rucksack. If my laptop has stayed at work then I sometimes put it in my rucksack, but mostly i carry it separately.