The problem is in the single lock mechanism that is designed to keep the lid shut.
As the bigger beaver apply pressure on the opposite side of the cage,,,,the aluminum frame flexes and allows the lock to disengage from the frame and the beaver can push it open.....

The simple modification is going to be installing double locks similar to what the "Hancock" trap has,,,,,,
This picture also shows the bent frames on the Koro.........

Here is a short video using the Koro baskets.......
using hancock and koro beaver basket Trap Trapping video