Echo Block

You can stand on it, grapple it, slope it with a hammer, actuate it and mine it back. The only thing missing is the picture. That combination is what makes it a building tool rather than a gimmick: platforms with no visible support, furniture that appears to float, housing that passes the wall check without showing a wall. Hiding a pool of liquid under it works while you are standing there, but liquids that overlap solid blocks get wiped when the world loads, so it does not survive a reload.

The Cyborg only stocks it while he is standing in a Graveyard, so you need enough tombstones near his house to flip the biome before anything appears in his list. Set that up once and buy in bulk. Spectre Goggles are the companion purchase, since without them you cannot see your own work while you are building with it.

Echo Coating does nearly the same thing to blocks you already placed and costs less per tile. Reach for the block itself only when you need something that was never there to begin with. For anything you are hiding, coat it instead.