Cutting with just the horizontal lines selected works fine. I've cut the object, separated it, isolated the left side and what I need to do here if figure out how to fill the hole. You said that I just have to planar it. I've tried that on the separated object, tried joining all the surfaces together and doing that, tried joining the individual sides so they form a half finished cube, and also tried selecting just the edges I want to planar, but it is not working for me. I haven't gotten it to fill a side even once. Is it really possible to fill the hole using Planar or would maybe using Lofting be a better choice here?