1952mike:
I think you should rethink your problem. The rubber pad will change the frequency and amplitude of the box vibration, but it probably won’t solve your box failure (cracking) problem. The bolt loads won’t be distributed very evenly to the rubber pads or the box, they are fairly concentrated, and that is exactly what is causing your low cycle failure (cracking of the box). But, now you sill have hard box, hard ½" bolts, to hard found., with a spring btwn. the box and found. You say nothing about the design of the box, pl. thick., joinery, location of bolts wrt side pls., etc. and the box is what’s cracking and failing. I think you should look at the design of the box, and more specifically how you put the bolt loads into it. While I know nothing of the box design and its C.G. v. vib., etc.; if you could distribute the bolt loads, in shear, up into the sides; rather than in bending into bottom pl., you would probably solve your problem. Move the bolts further apart will lessen their loading due to vib., then tighten down so box amplitude never completely unloads any bolt. You always want some prestress left, and rubber which might take a permanent set may not be your best choice. For example; put the bolts through a piece of 3" long 2" sq. tube, and weld that to the sides in the corners of the box, with a 4x4" block of rubber under each corner.