Then the coupler pocket of your choice can be added, glued to the styrene plate, and centered on the hole.

On this car, I used the #251 Kadee pocket that's included with the #158 couplers. They're nicer to look at than the traditional #5 pocket... with a buffer plate; and they're nicer to use with a snap-on lid. But they're not as realistic as a Moloco or Scale Accumate pocket. Kadee's own #261 would provide a narrower "scale" pocket... but I don't have any in stock at the moment.

Be sure to sight along the frame to verify that the pocket is both straight with the car, and centered with respect to the underframe. Set aside to dry.