A tip from my blue collar days, when I also only had a 20 minute lunch: eat a decent breakfast, and pack something like an apple/granola bar that you can quickly snack on at ~2-3 pm. If i try to wolf down food that quickly I just feel ill & sluggish, but eating a smaller lunch & a mid-afternoon snack actually left me feeling better. I continue to do this, it helps avoud afternoon sluggishness.

Although I would miss the rest of break time, I'd much rather work 0.5 hr later in the day and have a brief moment of respite.