It was time to upgrade this car to the new Sergent couplers. This photo shows the coupler installed.
Installing them on the MTH car is trivial. I unscrewed the Walthers ProtoMax. The new Sergent couplers were installed in a Kadee #802 coupler box. I then used two screws to install the coupler box. MTH provides the holes, so it is easy to install using these coupler boxes. Update: I have since switched back to using the Walthers ProtoMax couplers.