One of the cheapest and simplest and best looking couplers to install is the KD #153.  Gary White is using the HO #153 short shank “scale” couplers with whiskers from Kadee.  Gary installs the directly on the car without a coupler box.  He uses scale 4″x4″ board on the sides for the whiskers to press against.  The boards simulate the boards on the prototype cars.  Gary clarified how he installs his couplers “I should mention I don’t waste the coupler boxes, I cut them up and saw off the mounting pins and use those with the coupler so there is no shifting around on the mounting screws.”

On the left is a completed installation. On the right is a car with the scale 4″x4″ boards.

Same installations slightly different angle

Two cars with the HO scale couplers installed

For passenger cars, longer shank coupler may used and could be preferred.