FYI,
I have used strips of cork used for making gaskets for low-profile roadbed. You can get this material in auto parts stores; it comes in rolls and is quite is inexpensive. it’s about 1/8" thick, so if need be, you can use as many thicknesses as you’d like and make the next layer slightly more narrow than the bottom one. The ballast covers up the “stair-step” effect.
I would appreciate it if anyone has anything to add.
Thanks,
Tom