My turntable rack is solid cast concrete. https://systems.audiogon.com/systems/8367 The hardest part was making the mold with curves so it doesn't look like concrete. The thing about concrete, it is massive and stiff but not very well damped. So it should be combined with something to damp out the ringing. In my case the bottom and top shelves have a depression molded into them, filled with sand, so integral sand bed shelves. The whole thing was maybe $50, plus $150 for the granite top plate, awfully good for the money.
Even 750 lbs of concrete is not enough to insure absolute stability. Now recently this whole rack was put on Townshend Pods. Not easy but a real nice improvement. The equivalent budget move would be Nobsound springs. Awfully good for the money. Do not use cones. For only $30 Nobsound will be better than any cone. Use directly under the component, not under the concrete. For turntables you can try both ways and even both together. Which will be best depends on the situation.
Even 750 lbs of concrete is not enough to insure absolute stability. Now recently this whole rack was put on Townshend Pods. Not easy but a real nice improvement. The equivalent budget move would be Nobsound springs. Awfully good for the money. Do not use cones. For only $30 Nobsound will be better than any cone. Use directly under the component, not under the concrete. For turntables you can try both ways and even both together. Which will be best depends on the situation.