Exclude/Replace Mask in Preview

Hello,

We produce custom phone cases. Usually, design covers the front side and the edges of the case. In order to show edges, I created a mask with transparency 50% on edges, so users could see the printable area including edges. However, I want to show in preview the mask without edges where customers going to see just the case itself. I spent a lot of time trying to find a solution here but could find anything. How I can replace or exclude mask in the preview? 

Thank you!