Shape of the container and the number of possible energy states
For a sufficiently small container, the size and shape of the system is significant, e.g. some energy states are prohibited. For example, CdTe quantum dots behave differently from the bulk material.

However, the kinetic theory is based on the assumption that, "The number of molecules is so large that statistical treatment can be applied." The momentum is treated as a vector having x, y and z components and does not depend on the shape of the container (though a cube is convenient for illustration of three orthogonal directions). The equations can be derived using spherical coordinates, and they still must integrate collisions in all directions, so are equivalent.