The sealing frame, VW part # 113 117 323, is sometimes affectionately referred to by Type 2 mailing list members as the "Hoover bit" due to Bob Hoover's (see "The Sermons of Bob Hoover" review) enlightening explanation of its importance on a doghouse fan shroud engine. It mounts at the base of the doghouse cooler sealing an otherwise open area. Omitting it allows precious air to escape, reducing flow over the cooler. This vital tin piece has been obsoleted by VW, so if yours is missing or terminally rusty (as is typically the case), you should fabricate or locate a good used one.
Pictured below: To the left is an original, rusted sealing frame. To the right, a reproduction in aluminum angle fabricated by Dale Mueller.
 
 
 
 
 
   
more closeups:
 
 
 
 
 
   
Thanks to Dale for contributing photos of his sealing frame, as well as a dimensional drawing to help folks who are missing an original and would like to fabricate a replacement.
(For line drawings of the sealing frame relative to other cooling tin and hardware, see the engine cooling tin section of the appropriate VW parts microfiche at your Volkswagen dealer. A reproduction of these drawings can also be found in the appendices of the Gene Berg parts catalog.)