But not soft like pillow foam or the foam you would put on your bed like a pad. That stuff compresses and provides very little shock absorbtion. Much as it hurts, get the foam that is sold in your LHS. Pricy but it does the job.

On the other hand, I've found that if the tank is just jammed tight in the former hole and CANNOT move, that is enough. Put your rear brace in and you're done.