I used to use felt, but then I read somewhere that you shouldn't because when the gap opens the felt is exposed to the sun and can deteriorate to the point of failure.
I'm sure most would argue the minor sunlight is inconsequential, but it made enough sense to me that it could happen over 30+ years of exposure. I now use aluminum. Easy to cut pieces to size from leftovers. It only sucks keeping a stack of squares in your pocket because it guarantees you'll slice yourself at some point.
I'm not sure where I read this, but I think it might have been a Mike Guertin article. I'll see if I can dig it up when I have more time and not reading CT on my phone at lunch.