Hi,
in order to use a substance like rubber to dampen vibrations in a steel structure require that strain be transferred from the steel into the dampening
material where the vibration energy will be dissipated. Steel, being very stiff, will have very small strain displacements and even if the rubber is bonded to the steel
the displacement in the rubber will likewise be small and therefore of limited utility for dissipating energy.

Rigidity trumps vibration dampening every time. I would save the money that you might use on rubber by getting double thickness steel. If vibration is a real
concern to you use a good vibration dampening material like cast iron or epoxy granite.

I had the axis beds cast in iron, approx 115kg each, for my new build mill. I had hoped to use a cast SG iron frame of about 300kg, but I've maxed my
budget and have to consider making the frame out of steel. Steel is plenty stiff enough, and because it cuts and welds so well is highly versatile and modestly
priced.......but I still really want a cast SG frame.

I think I'll have to compromise, make a 'cheap and cheerful' steel frame now and in a year or so, when my budget recovers get an SG frame cast.

Craig