My mother gave me a very similar case when I started traveling for work. It was grey I wonder it its still around?
I have a case it once held a medical device, holds an AR, a full-size pistol and several mags for each. My foam job didn't come out as nice as yours. Its about half again as big as a normal briefcase. Its really my bug out case, innocuous enough but has whats needed just in case. I was going to make a spot for additional ammo. I think I made this close to ten years ago.
I have traveled with target pistols for years on the Sheriff's pistol team. I've seen every manner of damaged firearms due to the poor choice of interior padding.
I travel with two of the heavy air tight style lockable cases. One originally held expensive test equipment the other I purchased new. I wanted the interiors to match. After a little research I found that there is a huge difference between "foam" and instrument padding for travel cases.
You're on the right track in layering and cutting to fit. My target revolvers weighed 3.5 #'s and S/A 's <2.5 #'s. Without the proper approach and using the proper padding it wouldn't take muck to spoil an important match.
Keeping in mind that you need to keep ammo in the box for airline use of your case, I have an idea if you don't plan on using it for air travel....
How about cutting yourself holes in the pattern of the cylinder for the appropriate gun? You could then use the case as a "reloading block." An alternative is just buy a loading block and cut the foam for that, but that's not near as much fun!