I might give this 3mm foam a try.
Did you cut it with a hole in the center to match your action board?
I used a leather baffle that I made before as my template. Here is how I made the leather baffle:
I used a hollow punch to make individual holes in the baffle for each button.
Did you worry that you would make it such a tight fit that you would prevent air from passing at all? Did you cut little holes to let the air pass?
I found that the result was pretty air-tight so I added a small (about 3/16" dia) vent hole near the top of the leather baffle. This was enough to free up the bellows ability to get air.
Did you glue it to the inside of your grill?
No I used some small strips of sticky backed velcro. This has two effects (1) it raises the baffle off the end, and (2) made the baffle removable.
How would you say it affected your timbre, volume, and playability?
The leather version cut down the volume quite a lot but it also made the tone a bit muffled. After using it for a few weeks I decided that I did not like it.
The EVA foam only seems to reduce the volume, I don't notice a difference in tone. It does not quieten the concertina as much as the leather baffle did, but I am happy with it. I just wanted to quieten the left hand side lower reeds on an accordion reeded duet and not the whole concertina.
What color did you use?
Black! But you can get all sorts of colours.
There is a quality of my timbre that folks call shrill and can annoy, so I guess that I'm hoping it will cut down on the treble side of things as well as volume. Do you think I'll be disappointed or pleased?
If your recordings are anything to go by then you should ignore those naysayers. The tone on your Jeffries is just right for the music you play.
I think that you will be able to reduce the volume a little but keep the tone as it is.
But you could just buy some foam and experiment with it held against a speaker while it is playing one of your CDs. This might give you an idea on what it can do for you.
Whatever you do make it reversible.
I will add some in the next day or two.