Literally found this cart (just the bottom part) in an alley. It is 1 in. birch ply, with 800lb rated casters mounted on steel hat sections running the long way on the underside. It looked to me like a very beefy version of a bell cart, missing the frame that mounts to the four posts.
Steel pipe dropped into the existing sockets nicely and are secured with three fat set screws each. A top deck was cut from 3/4 in. ply, sanded and coated with black polyurethane, and mounted to the steel pipe risers with pipe flanges.
Steel bracing is provided to distribute the overturning moment of the mast and jib when loaded.
The mast will be hinged at the top deck, and fixed to the bottom deck with a removable pin, so it may be lowered to get it out of the room.
Two 12 in. long, swing-out outrigger jacks (more pipe) will be provided near the mast end, to resist the overturning moment when the loaded jib is swung around to the sides (ie; to lower a load onto the work surface).
I now have most of the steel and parts onhand except the mast itself. A swivel mount for the jib arm at the top of the mast still needs to be fabricated. Rather than a sleeve (hard to find), I'm planning a four-sided cage made of angle iron, with a COTS thrust bearing on top, and four machined brass bearing surfaces to take the loads on the sides of the mast inside the cage. It works in my head...
More as it happens.
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