I recently baby-proofed the upper level of the cage due to a mis-sexed pregnancy. Tinker had 3 pups, 2 boars and a sow which are 4 weeks old now (boars separated at 3 weeks).
I used the smaller mesh grids which came with the regular sized mesh grids (I think these are shelves to use in the cubes) and I also used some blue vinyl grids I have.