There are a few ways you can go about it. The easiest is to create a design with an empty space somewhere near the middle of the fuselage. Put your detaches and payload inside of there and then use rotators and panels to create the doors that cover the bays.
Finished the plane and is currently uploaded @TheLatentImage
Well it looks bad I started it like 2 hours@TheLatentImage
There are a few ways you can go about it. The easiest is to create a design with an empty space somewhere near the middle of the fuselage. Put your detaches and payload inside of there and then use rotators and panels to create the doors that cover the bays.