What I have done before, is make up the crumble topping and store in an airtight container. Make up the apple and berry mix and place in the cooking dish. Take both with you, along with the custard. When you get to lunch, put the apple and berry mix in the oven to cook. Wait until it has cooked for half the suggested time, then add the crumble topping. The crumble stays crisper if it's added halfway through the cooking time.
