I used some foam I had on hand a while back for an old, corroded 2470. The original diaphragm had opened due to corrosion as well. I had a 2425H dia on hand, so in it went. I put the driver into service and didn't think about it till today when I saw this thread. When I opened it to see if there was felt, I noticed that the foam I used (an egg crate pattern soft open cell type) had been pressed back by the diaphragm forming a ring indentation in the foam! I had put too thick of a foam in!
The interesting thing about this driver is that the center bump of the foam was actually pressing on the diaphragm as you might be able to see in my pics. For years I've heard how Ti sounds harsh, needs to be damped, etc, but ever since I changed a few of my drivers I did not think it sounded that much worse than the Aluminium I am still using. Today I found out why it did not bother me... Once I put thinner foam in I started hearing the harshness everyone has been talking about! Granted I did use a Constant Directivity curve on a driver mounted on a 2312, and cranked it up to SR level, but as soon as I put the thicker egg crate foam back the harshness mellowed out.
I checked my 2420s and they have the same foam pressing against the diaphragms, no wonder I like the 2425H dias in them! I feel it's a similar idea to the damping foam under some of the Ti dome tweeters, and it seems to work for me... If anyone wants a piece of this foam to try it themselves, I have enough to make about 4-6 more pieces.