I've created a "Smart Watch" model (with screen) in Blender 2.79. I used the "shrinkwrap" modifier over a square to create a rounded square, then added a flattened cube for the screen.
When I import my model into my game, the outside of the case is transparent to where I see the insides of my watch.
Everything I've found online about flipping Normals does not appear to work with a "modified" object. Solutions refer to menu options I simply don't have (or can't find.)
Likewise, applying a "Solidify" modifier (as another link suggests) absorbs my screen and watch stem/button into one big rounded cube (even though the only selected mesh is the case.)
How do I flip the Normals of just my shrinkwrapped cube so the textures on my imported watch face out instead of in? TIA.

I used the following YT video to create the rounded case:
Blender Tutorial: Create A Cube With Round Edges. The Easy Way.