I used to have a rack where sand was placed inside the structure of the rack (square pipe) and then I would insert a rugged construction rod inside it so it would be "suspended" by friction inside the sand. It didnt work, there was no advantage, vibration actually gets the sand more compact and the isolating effect dissapears.
For sand to do its job it should be in as free space as possible, hence the sand box idea.
About the Sorbotane, I am using the type they use to "glue" windshields in cars. I got it by the roll and just cut a bunch of stripes and place them under the wood, I left the waxed paper on it so it wouldnt stick to the rack shelve. It holds it shape nicely and works good for vibration.