Thickness I honestly couldn’t tell you. I used (most do) bicycle inner tube. I bought the Goodyear off road bike tire innertube and cut it down to be the thickness of the spring.
I am 99.9% sure you are right on the buffering. From the research I have done, Dodge and Ford alike have their systems buffer where as GM does not, so if we hit a "dead zone" it immediately goes out. This happens to me every time I go under and overpass or in some dead areas around where I live.
Just wanted to give an update. I did the rubber fix for mine this weekend, it worked. I used Goodyear bicycle inner tube. I will say it made me chuckle as I was underneath the truck sliding it in and read the big MADE IN MEXICO stamp on my spring and then the MADE IN USA on my inner tube. I would upload photos but it looks exactly as above and followed the same procedure as paumat.
I will take some pics this evening if I get around to doing it. If I do not then I will upload some pics Saturday. This better fix it. I am going to blow a gasket soon I am telling you, I had a 2002 Z71 200,000+ miles and the leaf springs never made a sound.
Worth a shot but to be honest, if they do decide to do warranty work you will have them greased (it won't fix it long term), then have them replaced (again not a long term), and then get fed up and do the rubber like everyone else has had to do. It makes me angry GM cannot admit the problem and get a fix out, either some better springs or a rubber not plastic silencer.
Update to this, clunk came back, I am putting rubber in today. That is the only fix that I have seen here on this forum and others that actually works. I am using inner tube from a bicycle tire. We will see what happens.
I have had the issue on my 2014 crew cab Z71. Went to the dealer, they did the grease "fix" lasted all of about 2 weeks. I went back, they installed new springs again the clunk returned. I got fed up and took it to a car wash, blew the springs with high pressure water to clean out all the grease they had packed in there. I went and bought a high quality dry graphite lube. I pried the springs and sprayed a liberal amount and let them dry. No clunk as of yet (couple days) but if it comes back then I will go with the putting bike tube in between them. It is amazing to me that a $50,000 truck can sound like it is 10 years old and been towing its whole life.