You want something with protein. Making your own protein bars is a great way to control portions; but I would honestly suggest something protein-heavy like meat, eggs, even dairy. Cheese is my favorite go-to between-meals snack, personally. ;)
A slice of whole-grain toast with peanut butter is super filling and packed with energy-boosting protein.
Here's some great protein-packed, portable snack tips: greatist.com/health/high-protein-snacks-po