I designed and built the HP O-400 to the same "good enough for the airshow" standard as the DH-4.

BTW I used the "filled with foam" technique on the DH-4 exhaust manifolds. After gluing the filled shells together I drilled out the ends next to the fuselage and glued in short lengths of 3/8" dowel. Then I attached the manifold to the fuselage with sheet metal screws through the fuselage and into the dowels. Lots less work than the Rube Goldberg method in the instructions.