What is an adjective that means that something is shaped like a cube?
Solution 1:
Cuboid describes a square-shaped object with six rectangular sides. No idea if that works with your poem, though.
If it was a cubicle that looks small from the outside but is larger within, it would be a tardis (think Dr. Who).
Solution 2:
If something is a square, then it is square! You could use the word tetragonal, derived from Greek *tetragonos (τετράγωνος) *If it is a cube, then it is cubic.