There are several books dedicated to using bentos for weight loss. I have quite a few of them, and most have a lot of good ideas. I only got Yaseru Obento Recipe recently, but it's already become my favorite bento-for-weight-loss book and one of my favorite bento books of any kind. It is in Japanese only, as are all good bento books unfortunately. But it's so full of great photos and illustrations that I think you could get a lot out of it even if you don't read Japanese.
Yaseru Obento Recipe just means "slimming bento recipes". The subtitle is kirei de oishii baransu bento ga atto iu ma ni dekiagari! dakara nagatsuzuki shite reboundo nashi! That's a bit long, but it means generally "beautiful and delicious balanced bentos that are made in a jiffy! So you can keep (making them) for a long time, and won't have a relapse!" That sounds good to me, and the book does deliver on that promise.