Once you have chosen to Add Cabinet/Panel in your existing room.

Flyover is found in Wall -> Panels & Fillers.

Enter the Width and Depth that the Flyover is meant to be.

NB. Height of Flyover is the thickness of the material.

Close dialog box to complete entry.

Once you have completed this item and entered any other items that may require ordering, to the room, then follow the process to order a room.