The basic Tamiya part found in the M4A3 kit suffers from excessively thick sections.  The advantage to using the kit part is that the interior curve of the lovers is captured in the Tamiya part.  The quickest solution to a decent looking part is to thin the plastic louvers down until they have the appearance of the sheet metal used on the real ones

BEFORE

This shows what is wrong with the kit part, and what caused the decision to modify it rather than build a new one from scratch.  The kit part louvers are extremely thick, nearly 3/4 of a scale inch, and look it.  The part is does have the advantage of actually having the curved inner surfaces of the real part represented.  A simple thinning operation is all that is really needed to fix the part.

THE REAL PART

Notice that the center section of louvers is set further in than the rest.

The next four photos are provided by Mike Foncannon.  Thank you very much, Sir!
These are from the Fort Knox M4A3 75mm wet stowage tank

Left:
An excellent shot of the prop.  The clip is barely visible at the far end.
Right:
Detail of the prop knuckle and the deflector construction.

Left:
Right side shot of the deflector showing the pivot.
Right:
Underneath the rear hull plate from the left hand side

AFTER

Left:
The finished resin casting with prop added
Right:
The plastic Tamiya part would look like this after a rework.
  Left:
The Tamiya kit part was reworked by thinning down the very thick sections.   A mold was made and the castings were used for models being constructed.  The four stages from unmodified kit to finished part are shown here.
Right:
The cast part as installed on the Tamiya M4A3.  The kit is missing the rear sand shield support and the plate between the rear hull plate overhang and the lower hull rear.
 

Mike Canaday
mmcalc at crosslink dot net
2-12-02