Chris, The best thing to use is an 'isopolymer cork' along the base, and a void filler (expended polystyrene will do) alternative strips along the base of your 'septic tank', then line the walls of your pit with a void filler, and a capping strip around the top with the cork material.
Seal it all up with duct tape and pour the inertia block. This leaves a 'floating' inertia block when the viod filler disintergrates.
Ive acheived measured 98% virbration attenuation on 3 ton (ram weight) hammers using this technique, its a 10th of the cost of 'spring box' foundation, the absolute best!
The 'cork' can be bought from Fabreeka, or James Walker ( TICO pads) - they charge you mega bucks if you get them to design the foundation (and they disclaim all liability as far as effectiveness goes
, but the materials arent to dear if you know what your asking for.
Hope this helps a little.