Which frequency gives the general shape of the squarewave grating.

You chose a frequency other than the fundamental frequency.

In the illustration below, both the luminance profiles of the fundamental frequency (a sinewave), the luminance profile of the squarewave grating, and the third harmonic. Notice how the general shape of the grating is already evident in the fundamental. The third harmonic (and all higher harmonics) cycle to fast to give the general shape of the grating. This is a general feature of fourier analysis. The lower frequencies give the general shape of the object being observerd.