Cessna 402 pilot reported the transponder altitude encoder unit became detached from its position during initial climb. Flight returned to departure airport.

Narrative

After taking off of runway XX in ZZZ and during the initial climb at approximately 400 ft; I felt pressure from something on the top of my right foot while on the rudder pedals. I informed that Captain who was pilot monitoring that I would need him to take controls once we got to a safe altitude because something was jammed between my right foot and the top of the footwell. He visually confirmed he could see something hanging down from the top of the well. We completed a three-way positive transfer of controls while climbing through 1000 ft AGL. The Captain took over radio duties as well so I could attempt to determine what the object was. Based on its appearance and the cables connected to the box we determine it was likely related to the avionics and would not pose an immediate danger. However; we determined it did pose a potential risk of jamming the rudder pedals if we continued the flight. We informed Approach that we needed to return to ZZZ. The Captain continued flying the airplane for the remainder of the flight and I held the box with my hand so that it and the wires were free of the rudders. We landed and taxied to the gate in ZZZ without incident. After shutdown we determined the item was the Transponder Altitude Encoder.