What you are doing there with the coupler is as probably as good as you'll get. It was just as well Kadee started making the whisker couplers, I had a number of Bachmann Silver series box cars etc some years back (must've been about the same time) and the copper centering spring with the KD#5's would not sit down into the cast in boxes. Only by the thickness of one side though. The whiskers solved that.Lately, I have been making the draft gear more to scale. This means a coupler barely fits through the opening and a #5 box is not even close to fitting. I have been using #158 couplers and just holding them in place with a screw and washer. The springs fit in the opening and keep the coupler centered.
Here is the setup on a Proto SD9. The coupler is mounted to a pad of styrene glued to the frame.
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