Charles A. Sherwood wrote:
be about 5 inchs square and about 3/4 inch thick. Please keep in
mind that looks are paramount for a model so slapping something
together with a heater core is out of the question.

Well... You could get get some Cerrobend, pour it into a sheet the
thickness of the spacing of the fins. Cut it into strips, and sandwich
the fin material between the strips. Mill and drill the assembly as a
block (make the holes undersize for a friction fit with the tubes).
Insert the tubes. The assembly should now be stable enough to solder the
tubes into the manifold while melting out the Cerrobend.

Kevin Gallimore