Adding a decal to an object with an opacity mask

Hi, I'm trying to add decals to a paper box with a window made using an opacity mask. I'm using the magic wand tool to select each side and individually add the decals for each side. This works OK except for the side with the window—when I try to apply the decal here, it gets applied to the inside of the box too. Is there a way to add the decal only to the exterior? Although I am selecting one side with the magic wand tool, once I drag the deal onto it, it seems like the whole box is selected. The only workaround I have found so far is to add the front decal to the base colour in Photoshop instead. 

Ideally, I would add the whole box design to the base colour in one piece, as I believe you can in software like Esko Studio. But the way the UVs come out, it's very fiddly trying to work out where the boundaries of each side of the box are.

Any suggestions to speed up and simplify the workflow would be much appreciated.

Thanks