One of the most impressive TT isolation boxes I've seen is Charlieboy's:
The box is made of solid maple pieces. Maple is very good for vibration control. The box is 2 inches thick. It measures 6" tall x 24" deep x by 32" wide. The box is screwed together then Ii caulked the inside so no sand could escape. The box itself weighs around 60lbs. Then i filled it almost to the top with 100lbs of fine washed beach sand that I bought at Lowes for about 4 bucks a 50lb bag. Then I had a 1" thick slab of granite cut to the inside demensions of the box.You might be able to take some of these ideas and incorporate it into a larger audio rack.