The front canopy frame gusset forms a seal between the forward and rear canopies to keep out the rain and to reduce wind noise in flight. The original plans call for a thin aluminum strip to be cut out by the builder to fit the curve of the canopy. I found a different solution at the Zenith builders site that is both easier to fabricate/install and also forms a better seal: Engine baffle gasket.
Originally designed to channel the flow of air through the engine cowling to the places that need the cooling air most, this thick rubber stripping is both durable and flexible. Today I fit a piece of 50mm baffling material to the aft frame of the forward canopy by using a punch through the existing holes. Easy peasy.
The baffling clecoed in place. Forms a tight seal with the rear canopy.
From the side. The baffling stays tight aginst the canopy surface for minimal drag.