It’s not unstable. The side panels are 12mm and the top 15mm. It’s rigid and heavy. There are cross braces at the back as well as the grey panels across the back. The top plate is heavy enough that it does not have to be screwed down, so it is effectively floating on gel. The castors have a vibration-isolating foot about 30mm in diameter and 20mm high. This Zacuda stuff provides good damping as well.
I made a turntable base just in case. The springs came from a pair of Townshend bars, the bars are under my subwoofer, for which they needed stiffer springs. This base does not wobble laterally and the springs compress to about half the height under load.
I could just put a pair of Townshend bars under this plinth, but they are ugly. I have a pair somewhere. It’s an option.
The idea was to have as much vibration treatment as reasonably possible. Wood and concrete both transmit vibrations with little damping.
One of the main purposes of this was to move the phono pre-amp as far from the power supplies as possible, hence the wide top plinth. The final version will be 700mm, this one is 750mm.
Most commercial racks are large and allow for ample ventilation. Only the silver unit generates any heat at all, so I could make it smaller and narrower than commercial units.