I'm planning to make some 2.5D cavern geomorphs out of cardboard to compliment my 3D Dwarven Forge Dungeon Tiles.
I plan to make each (well, most) geomorphs 4x4" in size with 2" openings to match the DF.
Here's a map of the tiles I plan to make --
Each large box is 4x4" square. I'm making curvy and straight passages. Some will have rough hewn sides (maybe all, depending upon creative license).
The large cavernous rooms *should* be expandible if I do them correctly. I think you can see how that might work.
I'll need about 2.5x3.5 feet of cardboard (no problem there) to make this set.
I'll keep you informed as it progresses!