The use of zippers wouldn't infringe on your copyright as long as they didn't try to specifically copy the Stillwater style.
Yup. According to the US Copyright Office, copyright protects recorded artistic works of many kinds. It does not designs, ideas, devices, or objects. However, the Stillwater trademark will prevent anyone from making an exact (or nearly exact) copy of the Nightstalker or from anyone else calling their cargo sporran "the Nightstalker."