Great job on your new cage! The second level can be either a 2x1 or a 2x2, wither will work. It all depends how big of a loft you feel like making and how much Coroplast/grids you have left. If you make a 1x2, there shouldn't be any need for more support for the upper level. If you make a 2x2, though, you might want to put a column (built out of Coroplast or a piece of pvc pipe) underneath the upper level to keep it a bit more stable. Nice work, and good luck on your new cage! =)