Mark Langford loaned me some molds to build VW cooling plenums. The quality of the molds is quite professional. Pulling parts from them was easy. Unfortunately they were not designed for the standard Revmaster intake manifolds but it looks like they would work with the "low-style GP" manifolds. They also do not support an enclosure for the "top-mounted" Type 3 oil cooler. The following photos depict what came from the molds when using 2 layers of 7781 E-glass cloth and AeroPoxy 2032/ 3660.

The plenum parts are loosely laid on top of a core Revmaster 2100D. Yellow masking tape is holding them in a rough approximation of the intended locations. I didn't go through final fit & finish as this was just an exercise to get initial measurements.

This is the 1 & 2 side detail.

This is the 3 & 4 side.