We bought a used 2015 model Mamaroo that did not come with the mobile arm. I've found that it's difficult to find that replacement part, so I made my own.

The armature itself is a scap piece of aluminum I had in the garage (I think it may have been a louver in a large vent) that matches the look of the Mamaroo rather well, but I suspect you could use just about any kind of bendable metal piece for that.