How to measure sit bone distance if I am very fat?

Many bike stores have a foam cube that you sit on for a few minutes. When you get up, the impression of your sit bones is etched in the foam and can be measured. The good bike stores will walk you through that before selling you a new saddle.

Here's how I measured my sit bone distance.

I bought a cheap "Gel" saddle from a department store. I rode it for a while. The cheap "Gel" padding in the department store bought saddle compressed permanently, leaving marks at where my sit bones touch the saddle.

The cheap "Gel" saddle thus confirmed that the most common saddle width, ~14 cm, is ideal for me. Then I bought a good quality saddle from a bicycle parts store in the same width.

Now, this may not be a practical way but it's interesting that such poor saddles that deform permanently are sold. The main problem in the practicality of this method is that no saddle is advertised by the padding permanently deforming.