Rubber will actually cause more stress on your mounting bolts and cause breakage. The best you can do is make sure your rear mount is flush to the tube, and your front mount is as solid as possible. Have a read through the thread 2Door posted on different techniques if you have an abnormal front tube.
If it's a fat tube you want is as solid as possible, here's one of my mounts with a normal U-bolt mount, double nutted:
I couldn't bring myself to drilling it