I used soft rubber bushings under the crossover board too. I suppose you could enlarge the screw hole and use rubber grommets top and bottom (like a compressor bushing). At some point, you just have to build it. The drive to hear them play just takes over

I stopped tweeking so I could start listening. My V's are on spikes front and back. They weigh about 200 LB. I suppose most vibration is drained to the floor. The liberal application of no-rez vinyl & foam made more difference than anything else. I also used inserts and machine screws instead of wood screws.
I think my pair is one of 3 pair of cabinets made with Baltic birch plywood by Jparkur. I picked the plywood because I have always liked the natural warmth of the material. I have considered granite in other builds as the material is reasonable now and the fabricators are hungry. I think the sound of granite in a loudspeaker might be cold and hard however. That's why I never persued it any further. I read somewhere that it is not a naturally damped material. I have seen it used on turntable plinths though