Let the side length of the square be a and the bottom length of the square of a cuboid be x (why the bottom of the square is relatively simple), and the volume can be obtained.

V=x*x*(a-x)/2

When the first derivative of v to x is zero, the extreme value is obtained.

Then x=2a/3.

That is to say, a small square with a side length of a/6 is cut at each of the four corners of the square, and then the cuboid without cover is assembled into the largest volume.