In theory concrete/epoxy filled tube works, reality is it is better than noting- not as good as a solid chunk of steel. You can rabbit hole different alloys if you really want to...
That said a 1” thick plate or um flat bar with some 1 1/2 xs pipe legs with some bracing (all welded) will be far more rigid and mo-better (tm) than any bolted and stacked structure in 1/4 wall tube.
Rectangle Tube cost more per pound than any other shape, and you want pounds. Sometimes the easiest and cheapest way is just old school real chunks of steel.
I did indeed consider solid steel plate, but in my reading it’s relying on sheer mass to dampen vibrations, rather than material qualities.
From what I’ve read, a tube structure is best for resisting torsion forces (
https://physics.stackexchange.com/questions/514821/is-a-square-tube-more-resistant-to-bending-than-a-round-tube#:~:text=Since a round bar doesn't really focus the,round tube will be the same for torsion.)
This is about square tube, specifically, but that’s a hundred-thirty or so bucks more in these dimensions. Rectangle seems like a reasonable compromise. And of course, thicker steel walls (like 3/8”) would be preferable, if not for the higher cost and weight.
Epoxy granite has proven excellent vibration-damping properties (hundreds of web hits, easily searched).
Furthermore, the amount and type of welding proposed (large amounts in very large chunks of steel) is beyond the capacity of my little 80 amp inverter. And I’m just not very good at making my welds look decent.
Given these factors, and a few others mentioned elsewhere, I’m still convinced my proposal is the simplest and most elegant solution within my reach. The internet can drop the steel tube and epoxy off in my driveway, and the rest of the stuff I can pick up on my way to work.
While I certainly wouldn’t turn my nose up at the sort of bench you’ve described, I’d need somebody else to do most of the work, and bring it to my house … and then promise to help me haul it around when the time comes to relocate !
