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

https://www.stall-bookmarks.win/upgrade-your-reclaimed-boxes-and-gift-bags-by-adding-sturdy-wire-handles-use

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