The present disclosure provides an apparatus and method for modifying the shape of a hollow structure. The method may comprise steps of providing a hollow structure having a cross-section with first and second diameters defining a first aspect ratio; heating at least a part of the hollow structure to at least its glass transition temperature, forming a malleable hollow structure; maintaining a positive pressure inside the malleable hollow structure to form a pressurized hollow structure; and pressing against a first side and an opposed second side of a heated part of the pressurized hollow structure, forming a hollow tabular structure having first and second opposed generally flat faces and a second aspect ratio greater than the first aspect ratio.