What mods to Porsche shroud to use in Type 1?

[wallace lee]

Jake,
in the other forum you mentioned that the Type 1 engine is 1/3 too small for the airflow of an 11 blade fan and unless the shroud is modified, too much airflow can be released resulting in power draw and ineffective cooling.

may i ask what mods are needed to be done to the shroud? Does this apply to the shrouds from Sharpbuilt,CSP etc? I thought they were bolt on.