PhilH
Western Thunderer
Somewhat better views of the tender rear coupling on one of my On3 locos, provision is made for the Kadee box in a slot in the underframe.
If you are going to fabricate a pocket on the rear of the underframe, this is the prototype coupler mounting from a drawing in Model Railroader.
This underside view of the loco may be of interest if you're considering re-gauging yours to On3
The loco is a limited edition version of the MMI loco as RGS #455 ATW (after the wreck) when it was rebuilt with a modified cab from a D&RGW standard gauge loco.