Composite structures and methods of forming composite structures are provided. A composite structure as disclosed herein incorporates one or more composite structure components, such as composite panels and composite inserts. A composite panel is formed from one or more sheets of fiber reinforced thermoplastic material. Composite inserts can include one or more composite blocks or braided sleeves. A composite block can be formed as a stacked or molded structure from trimmings or waste produced during the formation of the composite structures. A braided sleeve can include a seamless, woven sleeve formed of reinforcing fibers and thermoplastic threads. In a completed composite structure, composite inserts are at least partially disposed within a volume defined by surfaces of composite panels. The various composite structures and inserts can be given a final shape and can be fused to one another in a molding and fusing step.