I'd welcome your advice!
"make the top invisible"
- how do you do that? I don't see any options to change *part* of a furniture item.
"design the new top by drawing a room with a hole"
- how do you put a hole into the room?
"export-import it as furniture"
- Yay, I figured out this one, but for benefit of others this seems to do the job: Make a room and (still in the 2D view) select it, 3D View>Export to OBJ, Furniture>Import Furniture.
However

my new object created that way has no thickness. I can change the elevation off the floor, but changing the "height" seems to have no effect, it still has no thickness. Suggestions?
Thank you!