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.
This pulp rack has Sergent Couplers
Installation of the Sergent coupler from the bottom. This is the accurail coupler box. The freight car truck is an HO Tichy Archbar with a narrowed bolster and HOn3 wheelsets.
Two SR&RL freight cars with Sergent Couplers
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.
Sn2 Trains 
I never considered not using the coupler box as Gary has done. or any future cars I will not be using the coupler boxes. The major advantage of Sargets over Kadee is that there is no coupler slack- the slinky effect. And while they do not self center, neither does the prototype.