(Not in the Plans)
If you read the technical literature on the "NACA submerged inlet duct", you will find that the diffuser is a recommended element of the system. It extends the NACA duct ramp into the cowl area. It slows down the air to help build pressure inside the cowl. The diffuser is never mentioned in the plans, but you will see it being discussed in the CSA newsletter and in the canard email discussion groups. Some builders install one, some don't.
For me, the diffuser is a key element to my system! It forms the tunnel that feeds the incoming air from the NACA inlet directly to the engine plenum. It is enclosed on all four sides and it seals to the sump baffle. It air cannot escape to the firewall and wing roots. It must go to the engine and oil cooler.
My diffuser is built into the lower cowl. The forward flanges press against the firewall. The top aft edge incorporates a set of flanges that receive the sump baffle. To make it, I installed a pine stringer across the middle of the cowl. This stringer simulates the groove and the placement needed to interface with the sump baffle. I stacked urethane in the lower cowl and secured the foam blocks with pour foam. I placed a wood frame in front as a guide for matching the NACA opening at the firewall. Then I went about carving out the foam. I carved the diffuser "inside out", like carving the inside of a pumpkin. Once I had the shape established, I glassed it with 4-bid. After cure, I removed all the foam, installed the lower cowl onto the plane, and glassed the layups for the firewall flanges.
Next Steps --> Install the baffling seals in the groove.