Alexey Cherenkevich found a simple and quick way to align icons, he has the floor:

The bottom line is this: we make several viewports with different proportions, but with an equal area (by eye). In the picture, the viewports are translucent rectangles. I made three viewports: for a square icon, for an icon stretched horizontally and vertically. Now, depending on the proportions of the icon, I choose which rectangle to fit it into.

Everything.