C&C Why are cubes needed?

Well you could do something like that but it wouldn't be a c&c cage anymore, it would have to be made differently to make it work. The coroplast is just supposed to be for the bottom, then you fold the edges to make flaps so nothing gets out, then the sides are made of the cubes. The coroplast pieces you buy usually aren't big enough to still fold them to totally be the side of the cage, (though they come in a lot of different sizes so you might be able to find it, I've never seen coroplast quite that large though), so you'd have to use separate pieces for the sides, then you'd need to figure out a way to make it stay up and be secure, since normally that's what the grids and connectors of the cubes would do and you wouldn't have that. Also, not that it really matters too much, but the guinea pigs wouldn't be able to see out of the cage if the sides were totally solid.

So the guinea pigs can see out. They would not be able to see out of the cage if it was just tall coroplast.

Many people also use grids to make second floors, covers or bases for the cage.