If you want the most prototypical looking and operating couplers for your Sn2 train, consider the HO scale Type-E couplers by Sergent Engineering. Harry Downey has been putting Sergent couplers on his freight cars. Here are a few photos that he posted on the Sn2Trains@groups.io email group.
The Sergent couplers do not operated the same as KD style couplers. They operate more like the prototype. They do not auto center, so need to be aligned for couple and they require a magnetic wand to uncouple. They are not for everyone. Read more about them on the Sergent Engineering web site.
It might be reasonable to mount them without a box in a similar manner to how Gary White is mounting HO Scale Kadee #153 couplers. Without the coupler box, there would be more room for truck swing.
In addition to the Type-E Sergent Engineering offers a Sharon coupler. I’m not sure which is more correct.