You make a CHAIR.
A chair is a piece of furniture with a raised surface supported by legs, commonly used to seat a single person. Chairs are supported most often by four legs and have a back; however, a chair can have three legs or can have a different shape. Chairs are made of a wide variety of materials, ranging from wood to metal to synthetic material (e.g. plastic), and they may be padded or upholstered in various colors and fabr- yeah this is just a regular chair. Nothing special.
It seems like the dream-but-not-dreamself Karkat doesn't add anything to it.
Maybe it's because he doesn't exist? He seems like he exists, like he's just right there when you look at him, and he's between two other things that definitely exist, so it would make more sense for him to be there than not.
You find this all to be a little abstract and you'd prefer not to think about it too much.