Extracted the text below from a full-size aviation document. All common sense, and of course the principle are the same in models. The main difference as you are probably aware is the full size have mechanical doors which can be operated to increase outlet airflow at low speed.
It says is you need to use the high velocity air on the cowl outlet to draw the cooling air out while bearing in mind the much lower velocity air at the baffles which should be able to flow freely. On the full size radial, a smoother inlet airflow is provided by the adjacent cylinders. So i'm thinking it might be beneficial to create a curved surface ( like a pseudo cylinder ) in stead of a flat face which may tend to create turbulent air. You could paint a cylinder on it to improve the look.
Thats what i plan to do on the Stearman.
Three main avenues of approach to the problem of increasing the low baffle pressure drops existing at very low airplane speeds in order to obtain the velocity of cooling air necessary for increased heat rejection, are studied: 1. Use of the propeller slipstream to induce a depression after the baffle passages. 2. Use of the propeller slipstream to build up a pressure before the baffle passages.