What most all of us do is use the top of our "Fire Box" as the "Baffle Plate", allowing it to continue into the "Cooking Chamber" at the height of the top of the "Throat Opening" moving all the way to the other end leaving the same amount of opening as we did for the throat for the air to move into the area where the cooking grates are.
I hope that helps.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........