Harder and thicker boards are very durable for structural models. Complex surfaces can be made using construction similar to that of boat hulls with bulkheads to shape, hold, and attach the outer surfaces. Balsa airplane model technology is another way to think of constructing prototypes, employing bulkheads and structural ties that are inserted into cut slots. Models may be constructed as disassembly type, or glued at joints with surprisingly strong results. White glues work best with removable masking tape used as clamps. Another type of joint technique to consider is sheet-metal box type construction, most often associated with ductwork. Paper designs can be constructed with hems and flaps that may be taped together with double-sided tape. Extensions or attachments can be designed into the part and secured with slotted fittings made with a mat knife or hobby knife. Corrugated box construction should also be considered as a basic system of quickly fabricating basic, primitive geometric forms. Showing a corrugated box flat and unfolded will give the students clues to flap design that are very helpful.