I've heard a round (actually grout) underground vault. It had 4 18" pipe intersections. It was bell shaped. Two were blocked, top was a manhole cover. No bottom, just sat in the sand. This was not concrete, no rock, just fiberglass strands about 4-6" long and an epoxy/grout mix.
2 pro 15" drivers. It had an interior of 20 CF and 1500 pounds or so. It rang so bad, I had to spray the interior with expanding foam. It bassed up the whole 2 acre yard.
A column of Corian at 5/8 thick 18" x 20 x 72" with little bracing will hit a 1/2 ton.. pretty quick. Just a front baffle of @Corian and sides of MDF 550-600 lbs
I'm dealing with 410 lb each, NOW.. With HDF/MDF
Concrete is the heaviest because of the BIG rock, a lot of weight. Also tough to pump. Light grout mixes that are made with accelerators, can fluff to about HALF concretes weight. Fiberglass, Stainless all kinds of stuff go in the mixes.