Exactly the kind of thing that’s difficult to Google for prior art as well.
The best place to see prior art for this is not in terminal programs, but in tile-based video games. One example that I’m familiar with is “Chip’s Challenge”, which provided a set of “thin walls” in the tile set. These went even further than the OP, providing only three cells: wall on left, wall on bottom, and walls on left and bottom.
Of course you see this technique mainly in games written for memory-constrained platforms. (“Chip’s Challenge” was originally written to fit in a 128kB cartridge.)
This was/is a pretty common way of doing things in PETSCII on the various Commodore 8-bit machines.
McGugan boxes…. great name! There’s likely prior art all over the place but giving I have instantly committed this to memory.