I've got the plenum/baffling assembly partially assembled on my workbench. This angle might answer a few questions as to how it's all going together.  ...or not?

I've temporarily installed the basic plenum shells. You can see where I've started cutting on the left side intake. As you'll see in future pictures, I ended up cutting the left side up so that it looked like the right side--I cut the intake off. The alignment between my original mold and the cowling wasn't at all accurate.

OK. So fast forward about 6 months and I'm finally working on the plenum again. Wow, sometimes earning a living really gets in the way of my hobbies!

In any case, I used some of my handy-dandy hi-density foam block to as a basis for the mold to join my cowling intake to the plenum. I made a poster-board template of the cowling inlet. I then traced the template on the face of a foam block. I used a hacksaw blade to cut the shape of the intake into the block of foam. I then cut back about 1" to allow for a flat area that I'll eventually attach rubber or wet-suit material to that will bridge the gap between the cowling and the plenum. I shaped the foam block undersized, but basically in the correct shape.

The main purpose of the foam block was to act as filler so I wouldn't have to pile in that much molding clay!

This is the left side of the intake/plenum. I ended up cutting the foam back a little more on the inboard corner. As cool as it looked to have that nice bulbous radius, I needed the back end a little smaller so that I could mold in the flange that will attach to the aluminum flange. Keep looking at the next few pictures and this should make more sense.

OK. So you didn't have to use your imagination for very long. You can see that I've trimmed the inboard, aft section of the foam block back substantially as compared to the previous picture.

I covered the foam blocks with a mixture of epoxy resin and micro balloons. My goal was to try to seal the foam blocks a little bit so the modeling clay would stick better and so that the clay wouldn't get as contaminated by the foam.

Cutting to the chase a little bit here... I used modeling clay to smooth out and refine the shapes I wanted for the intake. My goal is to provide a constantly-opening intake so that the air will accelerate all the way to the cylinder fins.