Tim's solution is an excellent one that certainly gets my attention. The Athearn coupler boxes are so inherently unstable that replacing them alone is a giant leap forward. Like all the small couplers with smaller gathering faces, the Accumate Protos demand stabilty for good operation, if not good looks.
Although for a different car, Jim Singer tells me that the Accurail 40' boxcar underframe can be an excellent -and superior- replacement for that on the Athearn 40 footers. Now, don't interpret this as my saying that you should purchase an Accurail boxcar kit merely as an organ donor....
Denny S. Anspach MD