Try mixing a tablespoon or 2 of slippery elm powder with filtered water, and drink it 1/2 hr before eating. Its a good source of fibre, and kind of bland in taste so not disgusting. It helps fill you up. Its pretty cheap to buy from a health shop, and then you can put it into a small carry container, easy to put into your bag.
Also, chew your food more, and you will feel full quicker. I practice what Paul Mckenna suggests about after each mouthfull, put your fork down, chew thoroughly, swallow, then pick up the fork again.