Metal scouting turned up the “top-hat” riding shotgun in the big hole. Top-hat plays well with the cylinder, they have complementary visual weight and a nice interplay of round/flat.
The marble adhered to the steel backing bar, and is mounted and floating just above the ground.
The slurry mixer is hitched to the dump truck.
The curved Y hitch has a curved threaded bolt/nut with a pin: it is a unique form I found rooting around in an ancient store of military hardware. The female side has a weld hidden under the vehicle to a flush-mount square-tube welded to the twin chassis struts, just as it would be if this was a real vehicle.