Methods and structures for manufacturing and assembling matching parts in a manufacturing line are described. In some embodiments, the parts are portions of a consumer product, such as an electronic device. The matching parts are manufactured and assembled in a way such that the interface between the matching parts is substantially seamless. In some embodiments, the interface has a feature with curved portions. Methods involve measuring the dimensions of a primary part and custom machining a secondary part to seamlessly fit with the primary part. In this way, the secondary part is made to uniquely fit the primary part. The primary and secondary parts can be marked to identify them as matching parts in a manufacturing environment where numerous pairs of parts are made.