Thanks guys. This was one of those wonderful "free" mods, as I had this mat leftover from my Chevy Suburban where I had upgraded its console insert to the GMC Denali part. I just precisely measured, carefully cut with a sharp knife and a steel straightedge, then rounded off the corners with a scissors -- and voila, just dropped it into the cubby. No tape or anything is needed to secure it in place; its a perfectly snug fit just as-is, and this way I can easily remove it to wipe it clean of dust as needed.
I recommend! A piece of black felt could do the same thing if you don't have the right kind of rubber matting handy.
Cheers everyone,
Dave
Update - August 10, 2012: Based on the questions I got, here is how you can easily purchase your own console liner mat and make one of these yourself. I looked on the back side of the mat, and the General Motors part number is 15134788. Just walk into your local, friendly, Chevy or GMC dealership and have them get you one. Or, put that part number into google or ebay and you'll find a number of online GM parts vendors that you can purchase from. Less than $20 bucks. Good luck!