solid, port-able filler for intake manifold?

[Chris Conlon]

Hi,

For my next silly question, I'm looking for some putty or clay like
material to fill in large areas of an intake manifold. Once it hardens
I want to be able to come back with grinding tools and reshape it.
The reason is that I am having, hopefully, a custom manifold fabbed
up, and (a) it's lots cheaper to have it made very rectangular, and
(b) it might take several tries to get the porting right.

I am thinking maybe J-B Weld, in case lots, or polyester resin. Any
and all suggestions and warnings would be appreciated!

   TIA,
   Chris C.