I have 2 upside-down catfish (Synodontis nigriventris). They get max 4 inches, so pretty small especially for Synodontis cats, and would be perfect for a 40 gal. Many in that genus can get 8-12", so do your research first. Many in that genus are also called "upside-down", but this species are the most upside-down, spending about 90% of the time inverted. They have dark bellies and lighter backs, reverse countershading (normally dark top, light bottom), hence the scientific name "nigriventris" which means black belly.
They do not stay at the surface, though. I have heard they do this to eat mosquito larvae floating at the top, but I have never seen mine venture near the top. They stay close to decorations, like driftwood, large leaves, etc. They will also try to "hug" the heater, so something to cover it with might be a consideration as some people's have burned their bellies. They are also hard to observe, as they hide all day and only come out after lights out.
However, they are very peaceful and will eat almost anything! I think they're really neat and have considered getting a nocturnal light so I can see them better
Be careful, if you want this species, other Synodontis are often labeled as "upside-down catfish". If you want S. nigriventris, be sure they are brown with white dots and have very dark bellies.
good luck!