Just curious. What are you going to do with the empty couch space? Our couches are made into single beds. I had a custom mattress made to go over the entire couch. This make the crack much more comfortable for sleeping. The crack disappears with the mattress on top. So to speak.
Enjoy having space. I may put in a recliner. I will put some sort of chest of drawers thing over the pump and fill pipes.
I definitely dont want to carry a special mattress just to fill the gap.
I slept on the couch and was comfortable fine on the twin side, the other side of the gap gave room for my feet and legs to sprawl out.