There is going to be a gap on the underside of the magnetic pieces of the magnetic snap rivets because there is a "protrusion" that connects the prong to the magnetic surface, as shown below.
The protrusion will prevent the underside from laying flush against your fabric. However, this can be minimized by adding more layers with compression to provide a "cushion" that will cradle the protrusion, making it appear to sit more flush.
