Coupler started life as a Kadee #4. The box is completely home made. Styrene foundation, with a styrene top (also serves as a shim through the pilot draft gear opening to meet appropriate coupler height). See earlier photos for how the coupler shank was drilled ans shimmed. Coupler box was clearance drilled through the lid / shim, smaller hole drilled into the base block and the screw was self threaded into it.

I've done three this way; base block was glued to the underside of the pilot with CA cement in one case, styrene cement in two others. Neither has presented any problems.