I'm currently in the midst of working on a well-customized build, but I've come to a point on the case design where I'm not really sure what direction I want to take it. The build itself has an orange and black theme to it (note, not the same orange as what you see in the list of images below).
I like the way 2 and 6 look the most. the grate being darker helps give depth to the opening, and outlines it with some color. 6's stripe work pretty well with the geometry of the case.
I'd do a combination of the two, like this:
The gap between the bottom of the mesh cover and the stripe is about the same height as the gap between the mesh cover and the seam at the top.
And here's some battered sort of stuff for fun:
I sort of brought the scratches into the cover lining so it would look a bit more aged and grungy overall. The blue is a nice complementary color, but if you were to have a more neutral color, white would probably work.
Now that I think about it, a restored version of these grungy ones would look pretty nice too, because of the second color. I like the lining+stripe for just the orange, though.