Brett: The magazine on the Calico is that huge octagon block which is affixed atop of the receiver. It includes the rear sight and the crank on the back to tension the magazine spring.
The magazine assembly consists of two plastic shells (with integral guide ribs). In the center is the spring assembly which pushes the cartridges along the guide ribs. The shells are held together by a strip of spring steel on the bottom and a plastic strip which also has the rear sight.
While it shells can be machined out of aluminum, it would be far easier to cast it. Alternatively, if jigs could be made such that the integral ribs are properly spaced, they could be welded on. Casting seems to be the more feasible and easiest approach.
Hope this helps.