Let me show you all you can do inside this type of design:
The center deflector is most important optimization as it avoids the lateral airstreams colliding head on (it has to be done correctly as shown), but this shows other optimizations and bracing that can be done, You can figure out the length of the port parts with any calculator and/or sketchup (measuring along the centerline of the ducts ).
If your dimensions don't include the double baffle that's best, I placed the second layer on top of your original dimensions so the design above does not change, just place the second layer on the top if you want to do the bouble baffle, it's highly recommended.