Optimal waveguide width W for machined h=1mm air filled TE10 waveguide?

I need to design machined waveguide which is very similar to SIW (substrate integrated waveguide). Waveguide will be made from aluminum using CNC machining process, inner dimensions of waveguide is W x h, where height h is approximately 1mm. How should I chose width W dimension?
At first I calculated dimensions for waveguide rectangular cavity resonator, W x h x L, height of wavegude h=1mm is fixed and resonator inner walls have equal dimensions W=L, to obtain kind of symmetry of field distribution. I did not noticed such approach used for standard waveguides and other designs with custom waveguides. What should be considered when choosing W dimension? If width W is approximately equal to L of cavity resonator, then cutoff frequency and excitation mode is right, but there is still big room for W dimension tuning, which affects wavelength in waveguide. I would happy to see any comments on this topic.

Look into the operating frequency of the waveguide and choose the width so that the cut off for the fundamental mode falls below your frequency and not so small as to allow higher order modes to propagate. There are a number of waveguide and transmission line books available to describe this in more detail.

But there is still a room for width tuning, which gives different wavelengths: phase either shrinks or stretches along waveguide. Phase may stretch or shrink significant amount, especially in case of dfw (dielectric filled waveguide). So should one try to obtain symmetrical field distribution along W x L dimensions?