Many of us reading these posts remember our good friend Byron Rose, the RPA of Pittsburgh Scale Models, who offers arguably the best looking scale width coupler box in HO, manufactured in polyurethane resin. They are priced quite reasonable, and I think he sells them in packs of 5 pairs.
From: Tim O'Connor
Dennis I agree your Proto:HO coupler box looks great. I only
wish it were molded in polystyrene. It's not easy to work with
and mounting with screws is sometimes not possible.