I noticed that my wooden desk was acting like a sound board for my Printrbot. So I went to my local home center and bought some "softtouch 1/2 bumpers". They didn't look like they would stick too well to the rounded wooden surface. I found this part http://www.thingiverse.com/thing:24183 and decided the PrintrBot needed its own version.
I have redesigned this part. If you are considering this part check out the new one. http://www.thingiverse.com/thing:39300 I find it works much better in the long term.
Print out four of these things and stick some rubber/silicone bumpers on the bottom and install.