You can also use an insulation strip. It comes in different sizes, colors, and it's self adhesive.
I have also used some 3M double sided tape, with some green felt. It also comes in different sizes and thicknesses. I am using some now to put 60mm refractors in my Unitron clamshell.
You can also get very dense felt furniture foot pads (and rolled strips) at Lowes. It is also adhesive backed.