The reason MTL couplers can uncouple magnetically is the very free movement they have (they are loose in the coupler box). That also causes them to oscillate under certain condition (your slinky effect). If you tighten up the coupler box, that increases the amount of friction when the coupler shanks move in the box. That is how oscillation-dampening shock absorbent work. The ones in the cars from the early 20th century were actually friction based (as opposed to fluid based ones used in today's cars). The couplers will likely not uncouple magnetically, but they are free enough to swing and couple without excessive force. I have my doubts if they will be able to reliably magnetically uncouple (but you don't care about that anyway).
Sounds like your solution might be very handy for some anti-slinkers. 
Being a machinist since I was 14, I've been aware of something called "slop" between two mechanical moving parts for a very long time. I am going to bet that
@rodsup9000 's printed coupler boxes don't offer any more friction to the coupler halves, but since the central post is a bit bigger than the MTL central post, he's minimized the "slop" that's in the MTL design, meaning that the coupler won't be able to slide fore and aft as much as a stock MTL coupler box allows.
I don't know for sure, and since I don't have any of the printed coupler boxes to check, I'll go out on a limb and I'll bet that if they swing from side to side easily, that they'll still uncouple reliably using MTL's Magne Matic feature.
If the tolerances inside the 3D printed box are so tight that it impairs the amount the jaws will open up, then it could be that being able to push cars with the coupler jaws wide open may not work, but this hasn't got anything to do with the central post being less "sloppy" than MTL's design.
But, I might be wrong...and perhaps the "slop" engineered into the MTL design is there for a purpose.
I stopped using the Magne Matic feature decades ago, and have cut most of the magnetic dongles off of all of my N and MTL Z scale couplers over the years.
Now that I'm slowly converting to modified MTL True Scale couplers and retiring all of my Z-scale couplers, I'm not too worried about the dreaded slinky effect.
But, it is interesting that
@rodsup9000 's design eliminates the slinky effect. I would have thought that it would have taken more than a simple minuscule increase in the central post's diameter to do that.
Cheerio!
Bob Gilmore
