Turntable platforms

If you have a suitable mounting point such as a sturdy beam, hang it from the ceiling. 

The platform can be a thick, say 5cm plywood or butcher's block. Metal might also work.

This offers almost complete lateral isolation, and with a little tweak to the cables, vertical isolation. 

You could also consider a passive isolation platform such as those made by eg Thor Labs. These can be tuned to have around 0.5 Hz resonant frequency, and cost around $2,000+. 

They'd benefit from a solid floor but will still be sensitive to some degree to environmental ground noise, e.g. traffic. 

I've seen very good results with one of these, may take a bit of work to get it right, and whether it works better than a pendulum will depend on your situation. 

@geoffkait Springs, yes - but what design and how to damp?

Looks to me like Ligo is using coupled pendulums. Do those provide vertical isolation? Looks like that comes from the cables, and the intermediate masses?

@geoffkait 

So what are your solutions?

Undamped springs will ring. If the frequency's low enough it may not be a problem, but the ideal is to damp critically. 

I can't see any downside if damping is an option. It may be impractical to damp a pendulum. 

6 directions? Don't you mean 3?

How did you achieve 0.5hz vertical?