An idea strikes you.
You tie the rope into a loop, with a slip-knot at the end.
Whirling the rope around your head, you try to snare the chest.
After several throws, your loop goes over the chest and the slip-knot tightens around it.
You pull, and the chest shifts.
You pull once, and it falls over the edge and down into the trench
— but to your dismay, the weight of the box is enormous and it pulls you right over with it!
If you have a Levitation Spell, use it.