How to cut sorbothane?

I see a lot of misinformation here, some of it corrected. Sorbothane is designed to be compressed to a precise weight loading, which is directly correlated to its effectiveness (absorption coefficent); you have to use the Sorbothane calculator to determine the correct size for the weight you have. I doubt the Herbie Discs have any where near the same capability, but I have a few sets and still find them useful for many situations.  Sorbothane occasionally has a negative synergy on sound, whereas Herbie's Discs almost never do.

Use durometer 40, unless supporting something very light. Heavy duty sheers is probably the best tool. Cut squares, as you will not be able to cut accurate circles or use a punch (unless you can apply at least a few hundred pounds of pressure).

I experimented extensively with Sorbothane when I was starting out, using it under my components and even under my speakers (some products on eBay or Amazon, with names like SorboGel are not actual Sorbothane and are a waste of time). Sorbothane is the most economical absorber/isolator on a tight budget. As I got into higher end systems I moved to Townshend Pods and the adjustable Gyrotension Vega, which are much more expensive, but also deliver much more effective isolation.

Correction to my earlier post.  I recommend Durometer 70.  I don't know if you can buy it from McMaster (I bought mine directly from Sorbothane), but higher is better.  If you were able to cut the other with a punch you may be able to do it with 70, but the hardness is about double.

Someone referenced the relationship between Durometer and absorbing different frequencies, which is partly true.  All durometers absorb higher bass frequencies (50-100Hz) effectively.  Higher durometers will compress less with the same weight and have higher absorption coefficients in the 15-50Hz range.  Note that you can't compress the material more than 20% or it will internally cook and degrade over time.