So of course, I have modified it and my style is MORE cross arm guard than shoulder roll, although I have it as a part of my system
So for me you start in a basic guard, when the punch comes in rather than doing the full shoulder roll, I will flip my elbow up, to deflect it, and be ready to catch the punch with my elbow on the other side
Dustin Poirier does a decent example here:
He isn't at a perfect range to flip the jab up but he still succeeds in defending it
One of the advantages imo, of having more active elbows for defending is that because your arms are in motion a little more, you have more mental flow to also catch a kick if need be.
Here's a good sparring example:
The real difficulty with developing this, was just watching tons of tape for hours, rewinding etc, because every explanation of the cross arm guard for boxing and muay thai on YouTube is wrong. Cross arm videos are literally just people putting their arms up in crossed position to block punches and doing nothing else, often with the wrong arm on top lol
For me I mixed what I was being taught by my coach, with Archie Moore, who's my favourite classic boxer and have just developed it through trial and error with a few trusted peeps