You need to overlap and cable tie the roof grids to each other and then cable tie one side to the back wall only to act as a hinge - a roof for a 2x4 will need 3 grids per row. so to make a hinged roof (for access) for an entire 2x4 cage you need 12 grids. But, it is likely that a 3x4 will be too wide to roof without a central support, like a plank of wood, spanning the entire cage.
I hope that makes sense