Cut a white onion in half and toss it in the lunchbox and close it. Put it in a plastic bag and tie it up and place it outside in the sun for 24-48hrs. I do this when I forget to dump/clean a protein shake out of my blender bottle and it ends up smelling like Satan’s asshole. It’s the only method I’ve found that pulls the stink out of the plastic reliably.

I’m assuming the reason this works is a combination of the onion and UV rays hitting a see through plastic bottle. YMMV with a closed lunchbox. Might need to keep it open in a bag facing the sun instead if closed doesn’t work.