Swap out weak handles by adding 2-3 mm galvanized wire to your reclaimed boxes...

https://daisydustmaid.com/what-comfort-radius-fits-average-hands-for-a-wire-handle/

Swap out weak handles by adding 2-3 mm galvanized wire to your reclaimed boxes and bags. Drill 3-3.5 mm holes to thread the wire through, then twist and secure it tight