Cardboard is my cheap trick must be thin trace the clutch cover cut slide it in and done. It works great and absorbs oil, and it don't cost $$$ comes with the kit. I use the box that the carb comes in. Yea I'm cheap.
I drilled a bunch of holes in my cover.....They let the noise out so my cover won`t be loud....More into keeping my clutch cool than the noise it makes.....But thats just me, I might see if I can put rubba on it plus the holes see how it works out.....
Worth a try , yeah my cover was really ringing , it was amplifying the drive noise , I have heard others have had a similar problem , didnt cost me anything , had everything allready .