I would be wary of putting 2" of rubber under it, it will bounce - which is annoying to say the least, but will also result in a vertical hammering motion on the holding down bolts which will cause them to rip out or fail in no time.
If you are trying to attenuate the vibration I could write a thesis on it ! - the trick is plenty of mass, and having no point of your isolated interia block in contact with the surrounding ground - the vibration will find the contact point and transmit. If you google 'Fabreeka' it may show some isometric views of inertia blocks for a few ideas.
Sit down if you ask them for a price on any of the materials though
you can get away with a fairly thin slab, if you have a large contact area between the hammer and the slab - we scrape the timbers between hammer and slab to give as near 100% bedding as we can (think of a small contact area like a fullering die, it will break down the concrete / metal very quick !)