Here is where we're going: We want to be able to separate the air plenum sections by removing the pins from piano hinges - similar to what we and others have installed for connecting the upper and lower cowling.
We created a flat area on the bottom of the channel. The hinge will be offset so that the main plenum will be free to lift from the stub. On the top of the plenum, the offset will be opposite - again, the main channel will have to clear the stub. Along the side, you can see the line where the channel will eventually be cut.
The next step will be to pre-drill the holes for the screws. You can see the hole locations have been marked on the hinge. After the locator holes are drilled, we can make the cut line on this side of the channel.