I have tried such a setup on a normal sports CL. There was some noticable ram effect in the front part of the duct, because I had fuel mixture problems, which only were solved by shielding the carb bell from the airflow. The duct was close fitting for about 90 degrees fin circumference each side, and closer fitting at the rear than at the front, which had a generous radiussed inlet to slow down the air at cooling fin entry.
Upon landing, the engine always was cool to the touch.
The duct inlet on the Veco .20 (aluminium piston version) was 10mm wide, and ran from fin start to the top of the cylinder head, which was completely faired over. Air outlet was generous using side gills, to prevent pressure build-up of the heated air.