Welcome to the call making business Jon. An alum call is a good call, but they require a bit more maintenance to keep them playing, depending on what kind of alum surface you put in your call. What I've found with then is, the hardest part is to keep they clean, and keep your striker tip clean also, that is a must do with alum calls.
Anyway good luck at building one and post a pic of it if you do.
WI Youth & LTH Mentor
Dodge Co. NWTF